The term in actual universities is 'audit'. That's probably what I would recommend.
I would caution against selecting or rejecting terms based on connotations, because these are really culturally specific. For example, for me the term 'lurker' doesn't carry the negative sense it obviously does for many people.
I know that they use 'audit' in universities and in a sense it works because the prof sees the person. In the on-line world where your presence doesn't have to be made known to anyone - I think that is where lurker came into being (and not in negative sense). Personally lurker works better for me than spectator since a spectator usually has bought into the process i.e. spectators at a sporting event.
'Active Listener' ? - The issue of 'participation' (ie doing a post) is interesting because it is not always 'shyness' etc that holds people back - sometimes it is just being time poor. The 'like' button is one feature Facebook does really well. It is easy and fast to review what others are saying and hit the like button (which indicates yes I agree also shows I did read this / actively listen / consider this) and the then the ability to just make a comment - "we do that here too" or even 'interesting I'll look that up'. Also there isn't an automatic assumption that a comment on facebook will be as 'intense' (?) as one on a fourm like this ... just something else to consider :)