They will put up a search engine have no doubt about it.
It's a top-down culture in a company run by a doofus, where techs are asked to implement things based on decisions made by morons. That and the rush (as a knee-jerk response to Google indexing it) will likely ensure it'll be, um, ur, continue to be a viable market for independents.
But what do I actually know? - I basing that on recruitment and development rumours, some knowledge of what Google is doing, some comments made by money people about Facebook's finances - and some extrapolations I plucked out of my arse. Like - MySpace and Facebook, different dogs - same leg action.
I'm much more interested in Google+. Much smaller audience, harder to reach, not as impulsive at purchasing - but they've much more money. When I say smaller audience - Google+ is mostly real people, most of Facebook is bullshit and I'm not just talking about the accounts for fictional characters. I'd be more concerned about how soon the Facebook bubble is going to burst, than a Facebook search engine.