Google was the first search engine that was Good Enough that leapfrogging it became impractical. Facebook may well turn out to be the first social network to get to that level. Or maybe not.
eh, I think much of google's lead is also the infrastructure, and I don't mean the servers. If you showed up at my doorstep with a giant wad of cash and said "build a better search engine" well, about 2/3rds of the people I'd want to hire to help me are already working at google.
The rest are probably working on Bing, seems like Microsoft has a really talent team there, given the relatively small inroads they have made into market share shows just how hard it is to compete.
eh, If you wanted to do this, there was a real opportunity when Yahoo announced that they were going to use bing. There were many very good, very disappointed inktomi folks at Yahoo.