In the near future? Both. Facebook will see it's search expand as people spend more and more time there but Google as the default engine will continue to see significant activity for those doing research making it highly profitable.
It's likely that facebook will expand search and google will find a few social tools that hit but I don't see a short term scenario where one significantly intrudes into the other.
Of course that's the short term; history has shown us that sites rise and fall, but there's not much experience to look at so that's not a guarentee of the future.