Incentives don’t have to be monetary. Almost all “free” web services, including Facebook, only exist because they don’t need to charge for access. Users receive the inherent value of social networks (ease of communication and sharing, finding new friends, etcetera), and — in exchange — allow companies to monetise their information. This has been happening on the web for over a decade.
Facebook just needs to keep offering sufficient value, by bettering its products and features. The mere suggestion that Facebook should reward contributions to the site with money is preposterous.