EDIT: Ill add some detail to my point;
Google ramped up huge to where they are now, Facebook has been ramping up as well - but as Google enters maintenance mode with all of its service and tech - Facebook is still in a massively growth oriented position.
Even if they're new user adoption slows, or even stops - the number of products based on their data-set alone, for their 500MM users that can be built is huge - and the value of those products even larger.
This is why all the defections from Google have occurred.
They built an amazing foundation for the internet at Google, now they will build upon that with far more specific and deep information of the user base and how they are all related to each other
What facebook has is something that is required for the true "semantic web" which has been talked of for decades. Very very deep and specific information on every aspect of it's users interests. This is something that Google thought it was going to accumulate - but the problem is that from googles view, every person is a silo of interests - where they would have to do a lot of correlation on the back-end (which they have done) through their varied products.
Facebook, rather, has built a true interest portal and people are throwing ever single bit of human interaction data they can right into i - and linking it all up for facebook.
There is a very very interesting future (albeit scary) if facebook doesn't fuck this up.