5 key moments that changed Facebook

The addition of a chat-like interface to Facebook on April 6, 2008, was a natural and necessary evolution. Users had always sent messages to one another on Facebook, but by allowing those messages to pop up in a little window on top of whatever page you were viewing meant you could multi-task with less distraction.

It encouraged more conversational exchanges, instead of email-like missives, which ultimately meant people would spend more time on Facebook.

The evolution of chat also led to the arrival of Facebook's first mobile app dedicated to a single function: Messenger, launched on Oct. 19, 2011. Gunning for the likes of Microsoft's (MSFT) Skype and Google (GOOG) Talk, Facebook proved that its ambitions for its service went far beyond reconnecting old friends and sharing photos.