FB has a "network effect" the value of the network increases with the square of the participants (i.e. the number of connections you can have). Similar effects occur with human languages, telephones, and the internet. It's a kind of natural monopoly, because once one gets big enough, no alternative can catch up. That's why we have the internet, and not internets.

Google is different, because it's easy to switch search engines. One competitive advantage they have is "google suggest" - this takes massive capital investment in server farms to be fast enough, which is difficult for smaller competitors to match.

NOTE: in Google's opinion, FB was a very serious threat (hence google+), because more information about users means better targetted ads. So they don't think they're unassailable.