You simply can't know whether you're compatible with someone until you've actually met and shared some experiences. The term "online dating" is misleading. The date, if it happens at all, takes place in the real world.
Online dating sites must become more like the real world. We don't need long questionnaires; we need to interact and share experiences.
The idea is to provide opportunity to experience real-life situations, where both participants have to make decisions throught a discussion.