Twitter’s valuation has almost doubled in the last seven months, according to a Mashable.

Twitter is busy raising hundreds of millions of dollars, at a valuation as high as $7 billion. In December, the same company was valued at just $3.7 billion in a $200 million round of funding led by Kleiner Perkins Caufield & Byers.

Twitter, once a site without much of a business plan, is now a social media staple with a growing number of revenue streams.

Twitter introduced promoted tweets and trends last year, which proved to be the company’s first notable revenue source. Recently, eMarketer predicted that Twitter would triple its advertising revenue to $150 million in 2011. Last month we learned that Twitter plans to introduce promoted tweets into users’ timelines by the fall.

In October, it announced that it would develop a self-serve tool that would also allow small businesses to get in on the Twitter advertising game. The upcoming platform will function a bit like Google AdWords for Twitter.