In what world is Paper, Rooms, or Slingshot even remotely successful?
And obviously the messaging service attached to an already massive social network would be compelling, that's not really an "experimental app" by any definition.
In that case, we'll give you: News Feed, Pages, Groups, Messenger, Events, the Platform, and Connect, which have all been pivotal to Facebook's success (I guess more than one of these has been the first time email was dethroned as unquestionable medium for doing some thing or another).
