I'm considering using Jomsocial to create a social network and want to have some key people participate often. To incentivize them to do this, I would like to pay them based on their level of activity in Jomsocial. Is there anything in Jomsocial to help me do this?

Maybe you can use User points system?
documentation.jomsocial.com/wiki/User_Points_System
There is no predefined Activity Type for creating post on wall, as far as I can see, but creating one does not look too difficult.