regarding tipping element in social networks:
option1 - majority of participants trying to play kind ie receiving tips and tipping back to another users. In this case the same amount of money (injected to the system by some participants) just moving from 1 user to another - so money just represents some kind of appreciation which could be replaced with "likes" for ex.
option2 - rational behavior when participants want get more from activity than they spend. In this case whole network will be in