Also, facebook already have the traction - meaning, for their business model to work, they need to keep facebook users on the facebook platform.
By enticing/allowing their users to search from within facebook itself, Facebook will end up with such immense user demographic details prime for advertising space, that it is in fact Google that will struggle to compete (long term).
The next step would be a facebook browser. This would also lead to a mobile browser - allowing facebook to monetise their platform quite easily and readily. You would never need to leave facebook.... scarey.
http://www.extremetech.com/internet/92558-how-browsers-make-money-or-why-google-needs-firefox