I didn't make a specific post to come out on Fb but a discussion on there led to me mentioning I was transitioning. What followed was a discussion involving several of my friends who spotted it, and none of it was negative, in fact I found out most of them already knew at least one transsexual and none of these friends were trans themselves. The knowledge of it spread out from there as most of my friends there are mutual with each other and part of an online forum community for 12+ years. All family members friended on there are aware, probably through my mother (who isn't on Fb).
Only one person asked me why I'd put my gender on there as male and asked about it. That person was a peripheral acquaintance at best and he said nothing more after. This person had made a few passing overtures to me in the past so I'm not really surprised they noticed.
There was no backlash as such from anyone really. One person on there I'd been friends with for years struggled with it slightly, in the sense of asking some questions in a private chat, and again this person also had a crush on me in the past which had not been returned and I found he was more nursing a wound about that, and not really this new knowledge. So I'm not surprised. He disappeared for a while, then came back and started talking again, apologized for being out of contact for a while, which was generally his way anyway.
I've not had a negative experience with it being known on Facebook, but then, I don't have school acquaintances, all family, etc. on there as friends. I only befriend those on there who I call actual "friends", so I don't have to worry about the potential backlash from, say, an elderly Catholic relative I have or something like that. I know most people have a plethora of Fb "friends" who are not necessarily actual friends, and feel they have to watch what they say on there because of it. I don't really have that problem.