Not momentum. FB and Google lost their momentum a long time ago. What they have is power and market share. What google, facebook, etc are doing is no more different than what standard oil did in the late 1800s. Get big enough so that any other competitors has to either sell out or die out from underselling or external pressure ( manipulated railroad shipping prices, etc ).
Even if you have a good idea to compete with google or facebook, you aren't going to be able to get funding. Banks and VC aren't going to help you since many banks and VC are invested in facebook. Why invest in you when there are better opportunities out there.
Say you had a great search engine, but in order to get anywhere, you need to get "defaulted" on chrome or firefox. Good luck with that. Google is never going to let you be the "out-of-the-box" default on chrome. And you aren't going to outbid google for default status on firefox.
Once someone reaches monopoly status in a particular sector, it's hard to challenge them since they control almost every aspect of the environment. You either have to wait for the environment to change or move into another field. Or you need government intervention vis-a-vis window/internet explorer antitrust case.