Twitter Inc.'s initial public offering documents suggested a valuation of $12.8 billion for the microblogging service, underscoring the seven-year rise of a still-unprofitable company that has helped revolutionize how people share information.

In the most anticipated technology offering since Facebook Inc., San Francisco-based Twitter said it's seeking to raise $1 billion. Twitter pegged the fair value of its common stock at $20.62 a share in August. There are 620 million shares outstanding, according to people familiar with the company's financials, who asked not to be named because the number was not included in the filing.

The prospectus removes the veil of secrecy that had surrounded Twitter's financials since the company said on Sept. 12 that it had filed confidentially for an IPO. It shows how the microblogging service, founded in 2006, has evolved from a simple site for 140-character updates to a booming online-advertising business that generated revenue of $253.6 million in the first six months of this year.

"Whether it's worth $12 billion or not is really going to come down to how they can embrace this real-time news and information vision, how they can extend it to other revenue lines and how they can grow around the world," Brian Blau, a San Francisco-based technology analyst at Gartner Inc., said in an interview with Bloomberg.