Well yes, I don't have time to keep up with 10000000 car clubs via phone but on FB it's right there in front of me. Also, there are people who I care about enough to keep in touch with but not very often (old school friends etc) but FB takes the worrying out of that. I've found since I joined a few years back I've actually seen a lot more of some groups of friends as a direct result.
It's less about the 'announcing' for me as the social aspect. You can chat instantly online or via private messages, plus the advantage of being able to use it to advertise your latest venture.
I understand people's concerns and doubts, but in the right hands FB can be quite effective.