The Twitter micro-blogging service intends to mature into a formidable animal. Consequently, co-founder Biz Stone recently explained in his blog a new addition to the company’s aviary — Promoted Tweets. Translation: advertising.

It had to happen. Information may want to be “free,” but information providers and platforms don’t necessarily want to be, at least not forever.

Like other Tweets, the Promoted variety will be limited to 140 characters, and readers can respond to them or pass them along (“retweet”). The difference is, business and organizations pay to have placed at the top of relevant Twitter search results. They will be labeled as “promoted.”

Distinct from both traditional search advertising and more recent social advertising, promoted Tweets are start off as regular Tweets (they’re an “organic” part of regular Twitter). Promoted Tweets will also be timely, to connect the user in a real-time event, for example.

Much as Google does with unsuccessful AdWords, Twitter will drop a Tweet’s Promoted status if it doesn’t “resonate” with users (meaning they don’t respond to the Promoted Tweet in some way). Initial advertisers include Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America.