San Francisco, April 13 (DPA) Microblogging sensation Twitter is starting to turn its cachet into cash.

Twitter co-founder Biz Stone announced Tuesday that the service will now feature "sponsored tweets" at the top of some Twitter.com search pages.

Early advertisers include Best Buy, Virgin America and Starbucks, who will be paying to reach Twitter's estimated 22 million users in the US, and for the prestige of being associated with the trendy social network.

While some analysts value the privately-held startup at upwards of $1 billion, Twitter's main income so far has come from allowing Microsoft and Google access to its messages.

"Over the years, we've resisted introducing a traditional Web advertising model because we wanted to optimize for value before profit," said Stone, who in 2006 co-founded the site.

Stone said the advertising tweets would look like regular posting on a Twitter page, though they would be clearly marked as promotions.

"We hope you'll share in our enthusiasm as today we unveil a simple service we're calling Promoted Tweets," he said. "It's non-traditional, it's easy, and it makes a ton of sense for Twitter."

Stone chose to make the announcement via a Twitter blog. Normal Tweets are restricted to 140 characters, far too short for such an important piece of news.