Time is Twitter’s advantage. It’s also its Achilles heel. If you’ve got a time-restricted offer—say of discount hotel rooms if respondents book in the next six hours—Twitter’s your channel. Or if you want to float a last-minute promotion of breakfast pastries, there’s just no faster way to get a campaign up and running.

That is, as long as your biggest fans are watching their Twitter accounts within minutes of your tweet. Marketers who want to use Twitter’s real-time fire hose to get their messages out have to face the fact that for heavy Twitter users—and indeed for many average ones, depending on how many active users they follow—a marketing tweet will only stay at the top of the timeline for a short while, minutes to an hour. After that, it’s below the fold, buried under subsequent incoming messages where only the most diligent users will find and read it.

“Search” is the key word here. Twitter offers a search function at http://search.twitter.com that is far from perfect, but it’s at least one option for users who want to find out what’s been said most recently about a topic on the microblogging service. Others are also offering to let users search for tweets—including Google, Bing and a raft of third-party applications—but in an effort to start making money from at least some portion of its moon-shot user growth, Twitter has decided that the search function is the place to put the shovel in.

Twitter Search is a victim of the platform’s success, in that with so many tweeting firing off—50 million a day, according to the company’s own analytics—even a keyword search is likely to turn up just the thin top layer of content, especially if a user is searching on one of the day’s hot trending topics.

And the company is taking a leaf from the book that Google wrote to do it. As outlined a week ago and then elaborated upon in a developer conference in San Francisco later in the week, Promoted Tweets will let advertisers select the keywords they want their message to appear under. They will write the copy for a promotional tweet—standard Twitter stuff, 140 characters with a logo or graphic associated. And every time a user searches for that keyword, that marketer tweet will appear at the head of the results page.

That is, until Twitter determines that the tweet is not striking users as relevant enough. How will they tell that? With a “Resonance Score” that will monitor how often the tweet is re-tweeted, marked as a favorite, replied to or simply how often users click on any links. (Twitter maintains that other factors will weigh as well but isn’t giving further details to prevent gaming the system.) When a Promoted Tweet for a certain keyword starts to flag in terms of resonance, the next advertiser in line for that keyword will appear.

Right now that’s a limited pool of waiting marketers, because the Promoted Tweets system will only be open to a set of handpicked brands, among them Best Buy, Starbucks, Red Bull, Sony Pictures, Virgin America airline and cable channel Bravo. These early users will pay per thousand impressions, but it’s expected that once Twitter sees how users respond to the service, it may switch to an auction-based service to get more revenue from the most desirable keywords.

If that sounds like search marketing—short text messages delivered in response to keywords– it’s meant to. Google’s $20 billion pay-per-click business is a model worth emulating. But many of the questions that arise from the Promoted Tweets proposal involve the ways in which Twitter is not Google.

For one thing, chances are pretty strong that when users search for a term on Google, they’re looking to do research on a topic. And enough of that research is connected to an imminent purchase to make it worthwhile for brands to pay to have their search ads appear on the results page, complete with a call to action and a link to their Web sites.

But Twitter users are much more likely to be tuning in on an ongoing conversation about a topic: searches for “Paris” or “Hilton” are just as likely to be looking for the celebrity as for bargain travel or even tourism tips. So exactly will be the value of having a marketing message turn up in a search for those tweets? At the least, it’s unlikely Twitter will be able to charge anything like Google keyword prices, since the conversion rates will be much lower.

So the betting is that the brands who will be the first users of the Promoted Tweet platform will be searching to build awareness among specific groups of users—not necessarily to sell them something then and there, or even soon.

Then, too, lots of registered users employ Twitter without ever doing a search. So the reach of Promoted Tweets in this beta version is something of an unknown.

That may work out just fine, because the other question is how users will respond to finding marketing messages injected into the service that they’ve been using for bite-sized conversations both with their friends and with the users they follow. The result could be another outcry like the one social network Facebook created with the Beacon program, its 2007 attempt to monetize its own users by advising their friends of their recent purchases and other interactions on third-party sites. The book closed on that fiasco just last February with the approval of a settlement in a class-action privacy lawsuit by Facebook members.

The Promoted Tweets—notice, no mention of “ads” here—will be marked as “Promoted by Company XYZ” and marked off with a separate background color, and may run at the top of the search results but may not. Only one Promoted Tweet will show up in any given search result.

How many users will realize that these are actually paid placements is in some doubt. To this day, some indeterminate portion of users of Google and other search engines don’t realize that the shaded links served up in the OneBox at the top of the results page are “sponsored” in that companies have paid to appear there.

But let’s face it, most people are already quite aware that brands are taking an active part in the conversations that flow through Twitter. According to the company’s stats, about 20% of those 50 million tweets per day contain product or brand references. So it’s not like Promoted Tweets are invading a formerly commercial-free zone.

And the emphasis on “resonance” seems to indicate that Twitter is properly concerned about keeping the user experience valuable and not spammy. “There is not a single ‘ad’ in our Promoted Tweets platform that isn’t already an organic part of Twitter,” said co-founder Biz Stone in a post last week on the official Twitter blog. “This is distinct from both traditional search advertising and more recent social advertising. There is one big difference between a Promoted Tweet and a regular tweet. Promoted Tweets must meet a higher bar—they must resonate with users.”

When the Promoted Tweets program rolls out to other brands, it will behoove marketers to be equally concerned to keep their messages relevant, useful and interesting to Twitter’s user base, says David Berkowitz, digital media and online marketing strategist for agency 360i. To make effective use of Promoted Tweets, marketers will have to be able to fit them into the broader framework of a full-fledged campaign of Twitter conversations, not merely a one-shot ad buy.

“Twitter threw a really interesting wrench in when they made Promoted Tweets so heavily dependent on earned media,” he says. “We will not recommend promoted Tweets to any marketer who’s just looking to advertise on Twitter, and we will flat-out tell marketers not to set up a Twitter account just to be able to take part in the program. Not only do marketers need to be on Twitter and actively tweeting, but they need to be pretty good at it for Promoted Tweets to make sense at all.”

In announcing the Promoted Tweets program, Twitter COO Dick Costolo used an example that came up right after the Apple iPad went on sale earlier this month, when a large number of tweets using the term “iPad battery” came over the network, indicating that buyers were having issues getting their new devices to charge. Best Buy responded with a tweeted solution, but it quickly sank under the influx of new tweets and had to be refreshed every so often. With promoted Tweets, he said, Best Buy would now be able to keep that solution in front of everyone searching Twitter for a word-of-mouth answer to a hot tech question.

That capacity should make the program valuable for both marketers and end users, says Alisa Leonard-Hansen, social media strategist with digital agency iCrossing. “Up to this point, users have had difficult time finding information we’d like them to find,” she says. “I think there’s a good chance that brands will be able to surface and have more visibility in Twitter because of Promoted Tweets.”

But she agrees that finding the right strategy for using Promoted Tweets will be a learning process for brands simply because Twitter is not Google. Social networks have their own cultures and exist mainly to bring users together, not to sell products; brands are invited to the party, but they have to abide by the rules laid down by the majority of the members.

“Brands have been diligent about trying to abide by the rules of engagement in social media,” Leonard-Hansen says. “Whether on Facebook or Twitter, everyone has been respectful of the space, tried to engage in honest dialogue, be transparent and build a significant amount of trust.”

When it comes to turning earned media into paid media such as Promoted Tweets, both Twitter and the brands who market there will have to be careful not to abuse that trust by serving up irrelevant messages.

“Your keyword bidding strategy in Twitter will probably be different from what it is in Google,” Leonard-Hansen points out. “You’ll have to be more conscientious about what you’re bidding on in that space.”

Will Promoted Tweets finally answer the question of what the heck good Twitter can be for marketers? Not in this beta version, limited to Twitter Search, which is used by only a portion of the service’s 106 million registered users. But if, as projected, the program rolls beyond search into other areas of the service, such as individual users’ actual Twitter timelines, that could take Twitter out of the “new toy” box and place it more squarely in online marketers’ tool chests.

That would be good news for Twitter, which so far has racked up stratospheric user growth but has been at a loss to monetize that popularity.

The bad news? Google has just incorporated a “replay search” feature that lets users go back for months in looking for tweets around a keyword. If you’re looking for relative activity around a term, that’s a much more useful search function than anything Twitter provides on-platform. And Google knows something about selling ads against search.