Twitter Inc., the microblogging site that lets users post 140-character messages, is raising funding that values the startup at about $7 billion, said a person with knowledge of the matter.

The San Francisco-based company is in talks with investors to receive hundreds of millions of dollars, said the person, who asked not to be named because the discussions were private.

Twitter’s worth has almost doubled since December, when it received a $200 million investment led by Kleiner Perkins Caufield & Byers that valued the company at $3.7 billion. The startup was pegged at about $1 billion in 2009, a person familiar with the matter said at the time. SharesPost Inc., an exchange for shares of closely held companies, has assessed Twitter’s current worth at $6.8 billion.

Twitter is continuing to raise funds from private investors, rather joining other social-media companies in filing for an initial public offering. LinkedIn Corp., a professional networking site, went public in May. Since then, Groupon Inc., the biggest daily-deal site, and Zynga Inc., the top developer of Facebook games, have both filed for IPOs.

Matt Graves, a spokesman for Twitter, didn’t respond to requests for comment.

In June, Twitter co-founder Biz Stone announced that he’s stepping away from day-to-day duties at the company to join Evan Williams, another co-founder, in a new venture. The shift puts more focus on Dick Costolo, who became chief executive officer last October, and Jack Dorsey, a Twitter co-founder and former CEO who returned to an active role at the company this year.

Advertising sales on Twitter may more than triple to about $150 million this year, according to EMarketer Inc., a New York- based research firm. Still, that’s dwarfed by the growth of Facebook Inc, which brought in ad revenue of $1.86 billion last year, EMarketer estimates. (Bloomberg)

Twitter aims to get 1 billion users, Williams said in October. Facebook CEO Mark Zuckerberg had previously said that signing up a billion members is “almost a guarantee.”

Twitter revamped its site in September with the aim of being faster, easier to use, and better able to better handle photos and videos. The service now features a second column that lets users quickly look at messages without leaving a page.

Twitter’s new valuation was previously reported by the Wall Street Journal. 

(Bloomberg)