I'll copy my top-level coment here:

I think FB is well-positioned to provide a better online dating experience than specialized online dating platforms. Specialized online dating platforms have a disincentive to cater to people looking for stable relationships, since these people typically stop using the platform once they have found a match. For example, I find that OkCupid's redesign last year/earlier this year (I forget the exact time interval when) smells of a reorientation towards catering to heavy users of the platform. FB users, on the other hand, probably do not see (or see as much) a decrease in activity if/when they enter a stable relationship, and so FB can be more well-aligned with the relationship goals of its users.