(b) The mechanics of XMPP vs ActivityPub (ie point to point messages vs social media stuff) are very different.

(c) It's Facebook, and a lot of people really, really, _really_ don't trust Facebook, particularly on the privacy side. I think if this was, say, Tumblr/Automattic, or even Google bringing Buzz back from the dead, the knee-jerk reaction would be lesser, even if the potential _problems_ would be similar; Facebook just has a horribly compromised reputation (note that they _renamed_ themselves, which is not generally what you do when you have a well-regarded brand).