It's when you have a conversation through comments(on Myspace), wall posts(Facebook), or something of the kind.
A combination of comment and conversation.
Tends to be very slow, and very open, as anyone can see them.
These conversations can last for weeks, as you don't tend to answer immediately.
Here's a recent commversation between me and a friend John:
JOHN said on 5-1-01: HEY man, how's it going? haven't heard from you in a while!
YOU said on 5-2-01: not too shabby. Yourself?
JOHN said on 5-5-01: Just got home from the girlfriends. almost seven months together bro. Scary.