This is interesting. Up until now, I haven't really noticed this (truth be told, I was away for many months until now).
The magical formula seems to be "X + money". Anyone rememberes Tsu social network? It worked like a Facebook, but they shared some high percentage of the ad-revenue with users who generated the content. Nice idea, but you should see how it all tuned out:
Follow me and like 10 my posts, I'll do the same
..under every post/conversation. Yes, money changes the behaviour, but I'm not sure if for the better. Are we trying to solve the unsolvable?
Very interesting I hadn't heard of that site. I certainly think we are dealing with some very instinctual behaviours here!