Two words: Double opt-in.
As simple as it seems, that is the primary driver of the success of Tinder, helped generously by a smart on-campus initial seeding strategy for the customer base.
Every other dating site allows members to contact other members at the initiating member's will. All it takes is a few people with a dearth of tact and an overabundance of time to completely blow up (mostly women's) inboxes.
Basic free-rider problem: http://en.wikipedia.org/wiki/Free_rider_problem