I have never visited any teen chat rooms, like the one at Teensay.co.uk, but I guess my niece has visited a couple or so. Am not so sure if she chats with those she doesn't know, but I'm sure she chats with most of her former classmates there. It's one of the sites where she keeps in contact with most of her high school classmates. I think that's really great that they still keep in touch, since most of them are studying quite far form each other.
I remember, my former classmates and I used to keep in touch, too, but not through the internet. The internet then was quite unheard of (well, we know the internet was there, but we just had no easy access to it) so of course we communicated through snail mail. I used to make regular trips to the post office back then, and have grown quite familiar to the workers there. They were a nice lot and pretty friendly. But after the internet got more accessible I'm afraid I stopped writing letters frequently. Now, it's only an occasional card or two for some friends who don't have an email address. I guess sending emails is more convenient, specially if you have a computer and a reliable internet connection at home. :)