Twitter made an announcement today that the social networking site is now allowing companies to advertise in their search results.  Advertisers will pay a fee to appear at the top of search results as “Promoted Tweets”.

Twitter is a social networking site that allows its members to “tweet” about what they are doing in 140 characters or less. The site has grown exponentially since its launch in 2006, with celebrities, businesses, and politicians using the service to stay in touch with their followers.

The new advertising capabilities come as critics began to question the $1 billion dollar valuation of the site.  In a blog post, Twitter co-founder Biz Stone stated that the company took its time because they “wanted to optimize for value before profit.”

Over 22 million Americans used Twitter in March.

Stone say that Promoted Tweets must maintain interaction, meaning that the advertisement will disappear if it is not forwarded, or “re-tweeted”, by its members.