You could make people pay upfront and the more friends they enlist to join and stay active then the more money from their monthly/annual cost will go to charity

You could make people pay upfront and the more friends they enlist to join and stay active then the more money from their monthly/annual cost will go to charity, up to some limit. They pick the charity from a list. Or be blatant about it like some mobile games are. Watch up to 30 second videos (aka ads/commercials) to earn credits. Earn enough credits per month and no membership cost at all (have CC info captured for automatic payment though). Let users select topics of interest and let advertisers bid on their commercials.