In short, a percentage of all social transactions (tips, pips and kips).
The system can function stably at equillibrium by passing on proximal costs only but assuming the system is an attractive product then growth to equillibrium will need to be managed. A functional way to throttle growth is to take a percentage of all social transactions which will go entirely to and be the only source of net profit. The percentage value will be indexed to maintain a sustainable growth curve.
All business units will be run with a break even budget and the incurred costs will be passed on to users proximally and without markup.