Sorry, forgot to mention that...
These sites work because the offers that the user completes to get money/points are worth double for you.

For example a visitor comes to the site, sees that he can get whatever for free, signs up and does some offers. The advertisers pay the site for people to do those offers. The site pays the person 1/2 of what it makes from the person doing that offer. I cant really explain it, but heres a link: http://www.mivecmedia.com/

The site above explains the concept pretty well.

Hope that helped!