In case you were curious how they make money, here is BitBookie's answer:
"It's too early to monetize, but when we do, it will probably be through ads or affiliate programs.
I think that not charging a house take will have a positive effect on the quality of the site. Most sites do have a house take and that creates an incentive for them to be very pushy about driving bets. It's annoying and distracting. It's like being in a real casino!"