The announcement came via tweet on Twitter’s official account.

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 (@twitter) September 12, 2013

The filing followed closely on the heels of an acquisition of mobile advertising exchange MoPub for $350 million in stock; a move that could make Twitter a real competitor in social media advertising. In fact, Twitter’s recent traction as an advertising platform has been cited as the reason for it’s increasing value of late.

According to Forbes, The confidential filing means that Twitter’s annual revenue is less than $1 billion and that stock market conditions will be a major factor in the timing and price of the IPO.