Twitter said Thursday that it will begin placing advertisements known as \"Promoted Tweets\" in the timelines of users who follow a particular brand or company. Twitter said it will begin testing the new advertising offering with a number of companies including Dell, Gatorade, Groupon, JetBlue, LivingSocial, Microsoft, Red Bull, Starbucks and Virgin America. Promoted Tweets from non-profits such as the Make-A-Wish Foundation and the American Red Cross will also be shown, the San Francisco-based Twitter said in a blog post. \"When we decide to follow a favorite brand, business or charitable organization, we expect to be among the first to get a special announcement, access to exclusive content or a great offer,\" Twitter said. \"That\'s why starting today, we\'re introducing a way to ensure that the most important tweets from the organizations you follow reach you directly,\" it said. Twitter has enjoyed explosive growth since it was founded in 2006 but it is unclear how successful it has been in translating its popularity into profit. Twitter chief executive Dick Costolo, speaking at the Fortune Brainstorm Tech conference in Aspen, Colorado, last week declined to reveal whether the privately held company is profitable. But he said the number of advertisers on the platform is up 600 percent this year over last year, when it numbered in the hundreds. Twitter said the new advertising scheme will involve placing \"Promoted Tweets\" from accounts that a user follows \"at or near the top\" of their timeline, or stream of messages, when a user logs in. \"These Promoted Tweets will scroll through the timeline like any other tweet, and like regular tweets, they will appear in your timeline just once,\" Twitter said. \"Promoted Tweets can also be easily dismissed from your timeline with a single click,\" Twitter added. The advertiser-sponsored tweets will only be shown on the accounts of users of the Twitter.com website, not the scores of third-party applications used to access the service.