I don't think it's inherently rude or wrong, although I do think it can be done badly.
I kind of announced my engagement on Facebook. I first told everyone that I wanted to hear it from me directly, and then I changed my status to "engaged to" and posted a very brief status update. A few people congratulated me (most of whom were people I told in person, ironically), but the vast majority never said anything at all.
I think that things like Facebook announcements of anything personal ought to be limited to things where you're okay with a whole lot of people not saying anything at all, as well as people commenting on it. And, of course they should also be things that you think your friends list will actually want to know.
But I do think it can be a useful medium for this sort of thing, as others have said. A few people will probably fall into a gray area where they feel like they should say something, but weren't told personally and aren't sure how to respond. But most people will either already know, or will make a mental note of it and call it good. I also know that I would assume that if I found out about something through Facebook, most of the time, that means that I'm not really close enough to the person to need to say anything. I might mention it if I actually see the person and it comes up, but I would otherwise just say nothing at all.
