There is always a chance of that happening, however the fact that they have been so far a head of the competition since the begging I have to say I don't see it happening anytime soon.
nothing is impossible. if facebook have search engine option, then surely it gonna give some thrust to google.
I already use DuckDuckGo instead of Google for most of my searches. I have to admit that in terms of pure indexing capacity it will be very hard for a new company to beat Google, mainly because they have the best brains and billions of dollars invested in their infrastructure.
I believe no one can catch up with Google in recent years! It is not enough to focus on search engine to surpass Google! Google is not only a search engine now!
I dont think that this is a case of beating Google at what they do. When it comes to search, Google is too damn good at what they do. However, I think that the real thing that you need to think about more than anything is what Google does, and how people are finding things online. in 5 years time, are people still going to type what they are looking for into a box? Are they going to use recommendations through social media? If Google can meet people in that process, then cool, they will stay ahead of their game, however... as it stands at the moment, its not a matter of being good at search. Its a matter of being good at being where people want you to be. If that makes any sense at all...
Nothing remain constant on the earth. So nobody can predict it that Google will stay forever. But it would be so tough and really challenging to beat Google.