Not too long ago when iOS was released users jumped ship to Android due to the maps fiasco.

If Facebook forks Android, do you think people are willing to give up Google's ecosystem in exchange for Facebook's?

Even if you find Facebook chat / messages a viable replacement for Gtalk / Gmail, this still means no access to Google Play Store.

Amazon was willing to create a second competing marketplace and sell its devices as media players, but Facebook will not have those in place if they fork Android. Do you think Facebook has enough leverage to pull this off?

I don't think so, but since the phone is already in the works time will tell.