A business model has finally started to chirp at Twitter.

The popular microblogging site introduced Promoted Tweets this morning, letting sponsors pay for keyword placement in the site's search results. At least, that's how it will begin.

Twitter has lined up serious A-listers for the new advertising program, including Best Buy (NYSE: BBY), Red Bull, and Starbucks (Nasdaq: SBUX). I tried to bait an ad -- searching for "3-D TV" and "coffee" to see whether Best Buy or Starbucks would pop up -- but I came up empty.

There probably won't be any backlash over this phase of Twitter's monetization. If someone is searching for something, targeted search results bankrolled by an advertiser have long been standard on most engines. I'll be more intrigued to see what happens when Twitter tries to wedge these Promoted Tweets into individual users' personalized feeds.

"Before we roll out more phases, we want to get a better understanding of the resonance of Promoted Tweets, user experience and advertiser value," explains co-founder Biz Stone in this morning's blog entry. "Once this is done, we plan to allow Promoted Tweets to be shown by Twitter clients and other ecosystem partners and to expand beyond Twitter search, including displaying relevant Promoted Tweets in your timelines in a way that is useful to you."

This second phase will be crucial for Twitter. Will Twitter hold up when ads start sneaking into users' feeds?

We can hardly blame Twitter for trying to cash in. Ad-free Wikipedia and Craigslist may cultivate an altruistic allure, but in the ever-fickle dot-com world, Twitter would be blowing a golden opportunity if it declined to cash in on its current popularity.

More than 22 million unique visitors hit Twitter.com last month, according to traffic watcher comScore. The site has already licensed its real-time tweet results to Google (Nasdaq: GOOG), Microsoft (Nasdaq: MSFT), and Yahoo! (Nasdaq: YHOO), making online advertising is the next logical step.

Will Twitter soon jump the shark in 140 characters or less? I don't think so. In moderation, targeted ads provide a sound strategy. If anything, this new development should start making traditional search engines nervous. A financially motivated Twitter could be a fierce competitor.

What do you think of the arrival of online advertising on Twitter? Share your thoughts in the comment box below.