The bottom line is people are creachers of habbit. Most people dont even know that you can use a different browser than IE, just like they have no idea you can search for their shit on places other than Google. As for Facebook taking over Google? Never going to happen. They may someday make more money but If I am searching for the next best porn site or a script to run on my server do you think I am going to Facebook that?
Hundreds of Millions of people rely on Google everyday and will continue to for years to come just outa habit. Googles biggest problem will come from alienating their advertisers but if the bottom line suffers they will most certainly change their attitude towards their advertisers.