So, answering my own question : the only site in which I have ever taken an active part is Authonomy. Have I made any real friends? - yes, I think so - but only 6. Everyone else is just an online persona.
Why do these 6 people stand out? - The friendship thing went beyond the forum into the area of mutual help offsite. I value these contacts as much as any of my RL friends, and to return to my earlier question, 2 of these have already left Authonomy but we remain in contact.
I try to apply a couple of rules to my friendships:
Online - I don't do flirting
Offline - I don't do hurting.
If I regard you as a friend, I'm afraid you're stuck with me - WYSIWYG