I think Microsoft has already demonstrated that launching a successful new mobile platform is extremely difficult (if not impossible) at this point regardless of resources and hardware capabilities.
Facebook has leverage - if their future on iOS (and maybe even Android at some point) is at risk, they could very well release a Facebook phone and offer incentives to their users to move.
Developers would also be onboard if Facebook offers incentives to them. Despite all the bad reputation Facebook gets on the surface, most tech companies happily use and depend on Facebook for their marketing & advertising efforts, so they'd be more likely to embrace the Facebook ecosystem if iOS is interfering with their marketing ability.