It was only a matter of time, but Twitter has decided to delve into the world of advertising. While their user base has grown exponentially into the millions, their model wasn’t really a sustainable one, which was no secret to the company or its users. So here’s what’s going on, according to PC World:

Twitter will start testing a new advertising program that delivers contextually relevant ads in a user’s search results, the micro-blogging site announced on Tuesday. Called Promoted Tweets, the new ad program will post sponsored tweets at the top of your Twitter search results, based on the context of your search terms.

These sponsored search results, which are similar in some ways to Google’s AdSense program, are just the first step in Twitter’s new advertising plans; more changes will be considered after Twitter has had time to measure the success of Promoted Tweets.

Search Results

Twitter is hyping the "organic" nature of Promoted Tweets, where advertising will be based on tweets that Twitter’s advertising partners have already sent out in their regular Twitter streams. At launch, these advertising partners include Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America.

If a business wants a particular message to reach a wider audience, that tweet will become a Promoted Tweet and will appear at the top of contextually relevant search results. "There is not a single ‘ad’ in our Promoted Tweets platform that isn’t already an organic part of Twitter," wrote Twitter Co-Founder Biz Stone in a blog post announcing the program . "This is distinct from both traditional search advertising and more recent social advertising."

Twitter says the messages will be clearly labeled as "promoted" and will function just like any other tweet with the ability for users to retweet (repost), reply, or bookmark the message. In fact, what users do with these "Promoted Tweets" may be crucial to the success of Twitter’s new advertising program. The micro-blog says it will measure the success of particular tweets to make sure that only messages that "resonate with users" are included in Promoted Tweets. So if people don’t interact with a particular sponsored message by retweeting, replying, or bookmarking it, the company says the tweet will soon disappear from search results.