Altavista was dominant when Google entered the fray. MySpace was dominant when FB took off.
Whenever there is space for innovation and the incumbent does not react, a new player appears.
Facebook is my next to be toppled over bet. There is a huge space for innovation and FB is not nimble and quick to react nor it is particularly innovative on its own. Acquisitions and the social network barrier to entry protect FB, but they don't hold forever.