Twitter announces, in tweet, plans for IPO

"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," the company tweeted.

Moments later, in typically breezy style, the company tweeted a picture of its staffers, and said: "Now, back to work."

Talk of an initial public offering has circulated about Twitter for some time, and the Wall Street Journal estimated the company founded in 2006 is worth some $10 billion.

Lou Kerner at the Social Internet Fund, said he expects a big reaction from investors.

"Given Twitter's significant growth, and the market's love affair with social media, the Twitter IPO is posed to be a home run," he said in an email.

Twitter has become one of the fastest-growing and most influential social media services, used widely by celebrities, journalists, politicians and others.

The company said earlier this year it had "well over 200 million" active users, although some analysts say the figure is much higher.

Twitter this week said it was moving deeper into mobile advertising with the purchase of MoPub, a startup focusing on mobile ad exchanges.

The acquisition was announced late Monday. It was estimated to be worth some $350 million, according to the technology news site TechCrunch.

The fast-growing social network is expected to earn $582.8 million globally in ad revenue this year and nearly $1 billion next year, according to industry tracker eMarketer.

The San Francisco company is taking advantage of a rule adopted last year by the Securities and Exchange Commission which allows "emerging growth" companies with revenues of less than one billion to keep financial details confidential until they get closer to the IPO.

An analysis by the website Statistic Brain says Twitter has some 554 million active, registered users and is adding 135,000 daily.

In a sign of Twitter's growing influence, tweets have been analyzed en masse to measure happiness of cities and countries, and some scholars claim Twitter was an important factor in uprisings such as the Arab Spring.

Twitter is used to measure the popularity of TV shows, and is used by some programs for instant feedback.

"Twitter has firmly established itself as a social tool that people love to use and that the press loves to reference -- and more than 60 percent of US marketers are now on Twitter as well," said Nate Elliott, analyst at Forrester Research.

"And Twitter works hard to serve those marketers: the company learned how to sell sponsored tweets without upsetting users, when critics weren't sure it was possible, and it offers more useful marketing measurement tools than bigger competitors like Facebook."

But Elliott said Twitter need to do more to demonstrate it is a viable business.

"Marketers aren't yet fully satisfied with the results they're achieving on Twitter, and the company still has to improve its ad targeting and find additional ad formats to sell," he said.

"If this move helps them grow the business faster and focus on those challenges that'll be good news not just for Twitter but also for the marketers who use the site."

The company was started by Jack Dorsey who proposed the idea for Twitter while working with Biz Stone and Evan Williams at podcasting company Odeo. Dorsey later stepped aside to run another startup, Square.

A year ago, chief executive Dick Costolo said the time was not ripe for a Twitter IPO but indicated Twitter would remain independent.

Some of the concerns over social media companies have faded with Facebook, whose shares lost 50 percent after its May 2012 IPO, hitting an all-time high in recent days.

Facebook shares debuted at $38, and languished for months before recovering this year. Facebook hit a record above $45 this week, and closed Thursday at $44.75.

According to the analytics site Dashburst, at least 170 billion tweets have been posted since the company's launch in 2006, with users spending an average of 170 minutes on the site each month.

Dashburst estimates 80 percent of Twitter users access the site via a mobile device.

The most popular person on Twitter is Justin Bieber, with some 44 million followers, while Katy Perry has 42 million and Lady Gaga 40 million, all ahead of President Barack Obama at 36 million.

Twitter came to life on March 21, 2006 when co-founder Jack Dorsey's account (@jack) automatically sent out the first tweet, which read: "just setting up my twttr."

Dorsey followed it up with the first "human-generated" tweet: "inviting coworkers."

Since then, more than 170 billion tweets have gone out worldwide, and Twitter has 500 million users, according to Dashburst.com, a website that monitors the social media industry. Officially, Twitter has only said it has "well over 200 million" users.

Some more facts about a global phenomenon:

- Justin Bieber (@justinbieber) has the most followers with 44,418,729, as of 2200 GMT Thursday, followed by Katy Perry (@katyperry) with 42,603,233 and Lady Gaga (@ladygaga) with 40,094,580.

- US President Barack Obama (@barackobama) is fourth with 36,493,643 followers; the announcement of his re-election victory in November 2012 was re-tweeted 802,624 times.

- The average Twitter user has 208 followers and spends 170 minutes on the site every month.

-There are more than 200 million active Twitter users. Eighty percent of all users access Twitter on mobile devices.

- China is the country with the most Twitter users with 35.5 million, according to Dashburst.

- About 20 million Twitter accounts are believed to be fake.

- The hashtag (#) feature on Twitter which groups tweets by subject debuted in August 2007, proposed by a user.

- In October 2009, Google and Microsoft began integrating tweets into their search products.

- In January 2013, Twitter introduced Vine, which enabled users to make and tweet six-second looping videos. Some 12 million Vine videos are uploaded every day.

- Twitter is based in San Francisco, with additional employees in New York, Chicago, Los Angeles and Washington. Its total payroll exceeds 900.

- Twitter was incorporated in April 2007; it was co-founded by Biz Stone, Evan Williams and Jack Dorsey -- @biz, @ev and @jack.

- The initial Twitter logo was created by Stone, a former graphic designer.