January 2012: Facebook begins making Timeline mandatory.
February 2012: Facebook files for an initial public offering of stock. A few weeks later, it unveils new advertising opportunities for brands, allowing messages from them to mix in with Facebook status updates and photos.
April 2012: Facebook announces plans to buy Instagram, a photo-sharing social network, for $1 billion in cash and stock. It also discloses it plans to list its stock on the Nasdaq under the ticker symbol "FB."
May 2012: Facebook sets a price range of $28 to $35 for its IPO, then increases it to $34 to $38. On May 17, Facebook prices its IPO at $38 per share, and the stock begins trading the next day. The following week, the stock price starts dropping amid concerns about Facebook's ability to keep growing revenue and sell ads on mobile devices.