Well yeah, Google tried to build a social network (actually several) and failed; Facebook didn't try to build a search engine - that wouldn't fit into their business model of sucking you in and driving you to sign up to Facebook. I mean, after they bought Oculus, they even started requiring a FB account to use the device, but they couldn't have done that for a search engine, nobody would have used it. But lots of others did, and failed (or at least couldn't compete with Google).

So we can conclude that neither building a successful social network nor building a successful search engine is easy :)