Facebook i use for:
1. Contacting people who don't have MSN or who don't check their email
2. Contacting uni gliding club members and sending out mass messages (i'm the social sec)
3. Checking birthdays
4. Picture stalking. Everyone who has facebook does it, don't deny it.
5. Sharing pictures of meet ups with people, Flickr is awesome but for simple photo albums, facebook is good.
Twitter i fail to see the point in. Even Stephen Fry appears to have hit a new low on this one - i was surprised to see him (if it is really him) using "tho" a lot in his tweets. I have unlimited texts, i abuse the service when i need to if i need to talk to people urgently. Failing that i just call them.
My view on twitter is that it's like getting an RSS feed of everyone you know.
It's a good service if you're interested in what celebrities are up to - it's quite interesting to read Stephen Fry's every now and again. However, ordinary people are not celebrities, i wouldn't feel quite the same way if my mum started an account (though i belive my gran does ). And yes, as you say, you can keep up with events via it. But i have one word: IRC. It's been around longer, it's a proper "chatroom" and it updates in real time and is often more useful.