Recipe for good conversation: Listen. Ask questions. Pay attention to answers. Contribute when you can keep the conversation going.(And, I might add, be yourself. Not necessarily your whole self, everywhere, all the time...but nothing but yourself. People will trust you partly because you show you consider them trustworthy.)
Surprise #2: most of this is what I do already, face to face. I would never dream of walking into a room and telling everybody, "Listen to me because what I have to say is the most important thing"--so why would I walk online and do that? And on the positive side: I try to share information and ideas and make introductions that I thing people would benefit from. Does it really matter whether I do that face to face, on the phone, by email, or on Facebook or LinkedIn?