Facebook is already quite (but not perfectly—the big thing I've noticed is that the non-chat "messages" part of FB requires going into the FB app) well integrated into Windows Phone. And unless they have some truly groundbreaking interface ideas, I'd also rather see them continue to work more with MS on integrating into and pushing that platform, rather than introducing yet another one.

Also, does RIM have particularly nice phones, hardware-wise, these days? Another reason I'd rather see them work with MS is that then anything particularly cool they bring to the software side could run on gorgeous Nokia hardware.