The term "online dating" is misleading. On existing dating sites (LavaLife, Plenty of Fish, Match.com, etc) you don't date online at all. Rather, you search and email (and possibly IM) online. The date, if it happens at all, takes place in the real world... and is usually unsuccessful.*
The problem is, you simply can't know whether you're compatible with someone until you've actually met and shared some experiences. Yet online dating sites don't allow you to share experiences online. Rather, they simply grant you access to a database.
The Current (Pathetic) State of Online Dating
Dating sites gather basic data and allow users to sort potential matches by criteria such as: