None of them ever got so big as Google. Call it a phenomena like "Escape velocity." Google is so big, and has it's fingers in so many pies, it isn't going to get toppled unless governments attack it.
Aside from googles cash horde it really only has one other thing - it’s ad system. Its search engine and mail service is really a money sink and a way to drive people to their ads - it’s their ad system that everyone’s tied into and keeps the spice flowing.
Come up with an alternative ad system to deliver ads to web pages that’s more lucrative for the users and google will fold like the house of cards it is.