Honestly I'm really curious why no one has cracked it on facebook yet. There is a lot of potential there.
First, the privacy controls. If I am already Facebook friends with someone I fancy, we would just correpond via the messaging system - no app required. For an app to introduce me to a potential date whom I am not already friends with, we would both have to agree to let the other see our profiles. This agreement would be based on the Facebook mini profile - a photo and location. This sounds like speed dating without the speed date bit.
Second, segmentation of one's life. Facebook profiles are not anonymous blogs about one's sex life, or conversations with friends about a Saturday night party - they are public faces. Even in the real world, dating is done in private: alone, with few facts shared until the relationship is official. People are not going to want to muddy their public Facebook face with their dating life.
