We spoke about Facebook ages ago and there was talk about potential security problems, which I agree with. However I do use it a lot; I just don't install many of the stupid applications and games that pop-up. They're the trouble spots.
Anyway as to social value, that's why I use it! I found it's more dynamic than email to keep up with close friends. Lots of not so close friends are on there too, but we just message each other with anything relevant, but that's actually quite rare.
I always found that no matter how much people promise, when you leave a job or an area, no-one keeps in touch. Well I recently got made redundant and I'm very glad of Facebook now actually because I'm able to easily keep up casual relationships with old colleagues. I also find the "mutual friends" fascinating because on three occasions I've discovered some of my friends know each other but none of us ever realised the link. That's a nice surprise.
What Eric described is a problem I understand, but haven't had. People adding each other for the hell of it, like a competition or something. What I would say is that if you find people adding you that you'd rather not speak to, just ignore them and play ignorant that you don't really use it a lot and didn't realise they were there. Or you don't trust the silliness.
Actually I've done that by accident! I use a speeddial plug-in for Firefox and I've noticed it leaves me logged into Facebook if I go back to the homepage. I go back later on properly and find a long list of alerts from people I do actually like, but now think I'm an anti-social git because I wouldn't answer them!
I think part of the problem is that we grew up with email, thinking it was fantastic and we're quite happy to carry on. Kids today though think of email and even texts as old-fashioned (scary!) and find Facebook their natural home. Twitter is coming on very strong as well with similar consequences...
So anyway, you're both registered? Will you be my friend?