I've just been convinced to join Facebook.
The interface is very slick; AJAX where it's appropriate to avoid page reloads, asks intelligent questions, workflow after registering quite sensible. Grabbed contacts from my gmail account and added those who already have a facebook account as a friend; it offered to spam my 600+ other contacts, but it's easy to skip that (and it makes it very clear what it's about to do and warns you to consider if you really want to do it before you do anything).
But... Dave Winer has blogged about the fact that Facebook doesn't support adult relationships.
It also doesn't support anyone who lives outside a few select countries. I have to pass a captcha each time I try to add a friend. If I could verify my mobile number, I could bypass that - but that's only supported inside the US/Canada. Well done!
I can put my mobile number into my profile for my friends to see; but it insists on formatting it as an American number (614.071...) rather than the international number it really is (+61 407...). Not everyone lives in the US, not everyone should have their phone number listed that way.
My high school doesn't exist. I can suggest they add it, but it's in a town that also isn't in their list, and I can't suggest adding a town. So, I'm forced to go with "Queensland" as a town - a town that covers about 1/5th the land area of Australia. Brisbane, Bundaberg, both Coasts, and a few other cities (some barely bigger than my hometown) are listed - but not my hometown, and I can't add it.
It won't believe that I actually attended the uni I claim to have attended, because I don't have an email address in that uni's domain any more. Well, I should say "college", because it forces me to use the US terminology - even though "College" has a very different meaning here.
I can list my website; however, when I choose to change the privacy setting for this link from "Only my friends" to "All my networks and friends", it then reloads the whole page - losing, along the way, my website, my phone number, and other information I'd already filled in.
I can't enter my current workplace, because it doesn't exist in their database. I can't enter previous workplaces, as I no longer have email addresses there.
Not very impressed, so far. LinkedIn does a much better job of dealing with adult relationships and workplaces.