I do think Michelle was rude to post the information, especially so soon after hearing it herself. I don't think it fundamentally has anything to do with Facebook or posting it from the POV of her daughter, though--like I said, there have always been gossips and people who spilled other people's news, they can just do it faster to a wider audience now, and in cutesy multimedia ways.
I get that Jeff and Allie were just super-excited and not thinking "communication strategy," but it sounds like Michelle has a history of doing this sort of thing. She was rude but not surprising, in other words. In the future I think the best response is to just dry up the info supply, both to Michelle and to anyone who would tell her. A well-meaning person might accidentally jump the gun or get their wires crossed once, and you could say to them, "Just so you know, I was disappointed you did that, because I was looking forward to telling people myself," and they would be like, "Crud monkeys, I'm so sorry, I wasn't thinking!" and they'd never do it again. But someone who does it repeatedly--while not connecting it to the idea of wanting to control their own
information--I just don't see the point of saying anything to them. I just don't think they'd get it.
Story: At a staff meeting once, it was announced that a co-worker, Jenny, was pregnant. She was probably at least four months along, as we had already begun to suspect. After the meeting, I emailed some former co-workers who I thought would be interested in the news, who I thought wouldn't find out any other way. At least one of them emailed Jenny with congrats, and she got mad at me--she had been planning to tell that particular person herself, as they had remained in contact. I see now that since it was her
news, I shouldn't have taken it upon myself to spread it, no matter what (or at least asked her first). But, I had reasoned that since she was announcing it at the staff meeting, she now considered it "public knowledge," and had already told anyone she wanted to tell personally. Obviously this wasn't the case, which didn't make sense to me; but what I learned was that it making sense to me
isn't the point, I should have checked with her