Microblogging site Twitter unveiled its advertisement model on Tuesday, known as "Promoted Tweets". Promoted Tweets are ordinary Tweets that businesses and organizations want to highlight to a wider group of users.

The program will will be rolled out to two to ten percent of users via search on Twitter.com beginning Tuesday, according to the company.

Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America have already signed up with the new advertising program.

Users will start to see Tweets promoted by Twitter's partner advertisers called out at the top of some Twitter.com search results pages. Promoted Tweets will be clearly labeled as "promoted" when an advertiser is paying, but in every other respect they will first exist as regular Tweets and will be organically sent to the timelines of those who follow a brand. Promoted Tweets will also retain all the functionality of a regular Tweet including replying, Retweeting, and favoriting. Only one Promoted Tweet will be displayed on the search results page, Twitter added.

In the near future, Twitter also plans to allow Promoted Tweets to be shown by Twitter clients and to expand beyond Twitter search, including displaying relevant Promoted Tweets.

Twitter whose short text messages or "tweets" have become a global social phenomenon, is a privately held company. It does not report earnings, but its website says: "While our business model is in a research phase, we spend more money than we make."