"Persons that give much to others try to get much from them, and persons that get much from others are under pressure to give much to them. This process of influence tends to work out at equilibrium to a balance in the exchanges. For a person in an exchange, what he gives may be a cost to him, just as what he gets may be a reward, and his behavior changes less as the difference of the two, profit, tends to a maximum ("Theories Used in Research"). "
In other words we keep a kind of bank account of inputs and outputs in our relationships, and our viral activity and recommendations are reflective of that.
In fact it would be great to see what the balance of communications are on some people's Facebook accounts. Are they getting more messages than they are sending, or vice versa?
Other research in equity theory shows the people who are the most satisfied with a relationship are the ones who feel the relationship is equitable. If you get too little, you feel cheated, and if you take too much, you feel guilt as the chart below shows.