I think their incentive to be successful is word-of-mouth advertising. I'd never considered online dating until both my aunt and a couple at work had success with it. I tried their service and also met someone I very nearly married.

Granted, I've never paid for a dating service. But the business model is much like Facebook's: get a bunch of free users to voluntarily give personal data about themselves, then sell that data to power users. There's a good degree of faith that online dating works in general (which is bolstered by successful matches), and some people are willing to pay for an advantage in finding someone (and in being found).