SAN FRANCISCO -- The most anticipated Silicon Valley initial public offering since Facebook is in the pipeline: San Francisco-based Twitter announced Thursday that it has privately filed for its first public sale of shares.

In a message sent on the corporate account of its own microblogging service, Twitter said Thursday afternoon, "We've confidentially submitted an S-1 to the SEC for a planned IPO. This Tweet does not constitute an offer of any securities for sale."

Twitter was founded in 2006 by Jack Dorsey, Ev Williams and Biz Stone, and has grown to more than 200 million users who share their thoughts in bursts of no more than 140 characters at least once a month, with estimates of total users surpassing 500 million.

Twitter's massive growth has produced strong IPO buzz for years, but neighbor Facebook's rough Wall Street debut quieted much of the excitement. The Menlo Park company exercised a record-breaking IPO in May 2012 that valued CEO Mark Zuckerberg's creation at more than $100 billion, but problems with initial trades and doubts about Facebook's ability to generate revenues helped push shares from an initial price of $38 to less than $20 in the first year of public availability.

Facebook stock has rebounded of late, however, pushing to an all-time high of more than $45 Wednesday after the company's most recent earnings report showed strong gains in mobile revenues. The IPO market has also fully rebounded after a lull following Facebook's debut, with seven Bay Area companies filing to go public in August.

"There's more of an appetite for IPOs than there's been in some time," Thomas Kellerman, an attorney with Morgan Lewis in Palo Alto who co-chairs the firm's tech practice, told The Mercury News last month.

Twitter is allowed to privately file its IPO paperwork for review before it is openly released because of new federal rules that allow the practice, which gives the Securities and Exchange Commission time to request changes away from public view. Such private filings are allowed only for companies with less than $1 billion in annual revenues.

"These companies can have discussions with potential investors, refine their story, await next quarter results -- all without having to do so under public scrutiny, which can provide pressure to get the deal done," David Golden, a longtime valley investment banker who's now a partner at AOL founder Steve Case's Revolution Ventures, said recently.

Golden added that Facebook's recently having climbed above its IPO price has helped goose investor confidence in consumer startups.

While the company doesn't disclose revenue, analysts estimate it's well below the $1 billion threshold. The research firm eMarketer projected Twitter will have about $580 million in online ad sales this year, or more than double eMarketer's estimate of $288 million in 2012. The research firm projects Twitter's ad sales could rise to $1 billion next year.

Like Facebook, Twitter generates its revenues from advertisements, with businesses paying to promote their tweets in users' timelines on personal computers and mobile devices. The company has taken pains to develop its revenue generation in the past year by opening its ad platform to third-party software, improving ad targeting with the use of cookies and data-mining, and partnering with WPP, the world's largest advertising agency. Earlier this week, Twitter announced the planned acquisition of MoPub, a mobile advertising company, and said it will use the company's technology to offer advertisers real-time auctions for purchasing ads.

Bloomberg News reported that Goldman Sachs would be the lead underwriter for the offering, based on an unnamed source.

Staff writers Peter Delevett and Brandon Bailey contributed to this report. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/jowens510.