That is because you probably think that most people are like you and me. Interested in the internet.
That is not the case. Most people are interested in stuff like mountain biking, kayaking, photography, cooking, their friends, partying and so on. They couldn't care less where they get their information from. They are in that regards, lemmings. They don't even know that they can search for an answer to an elusive error message. Hell they don't even know what it is.
Google couldn't survive on google maps alone so if they loose the preferred search "war" they are in deep trouble.
Also mind you that Facebook is capable of delivering far more precisely targeted advertising in context of not only your interest, but your current situation at any given day (at least potentially)
It's easy to think that the rest of the world thinks like us, but they don't.
Case in pint. The recent skype re-design that drew a lot of bad reviews are all made by techies with at least a hundred skype contacts.
Yet 90% of people on skype have 5 contacts, 95% use the message window to write "Can I call you"
So all in all I would say that Facebook is in great shape to compete with Google. It's not made for the 20% but the 80%.