I don't see exactly how Facebook competes in search, all I see it as is an increasingly spammy social network that I could easily live without. Google on the other hand, is indespensible. The very sight of Bing makes me want to punch the monitor (when I'm not using my own PC).
The problem is, they wouldn't have the vision to isolate the solid WebOS development team from the rest of MS. They would tangle it up in departmental meetings and company-wide politics and cross-market product ties ('search your phone emails with Bing Mobile Edition Professional Basic!'). And then, besides being the underdog, it would also suck.