Or you could operate more like the credit bureaus do for banks by selling your service directly to websites instead of end users. You connect in the back end with all the sites, they send you users information, and then you parse it all into one solidified reputation.
Doing something like this, you could act as a sort of social network background check, as users would not even need to opt-in to your service, they would automatically be signed up when they join the various sites.
That's a pretty cool idea. A web-of-trust problem would have to be solved: the various systems that connect to the system would need to trust each other's internal reputation systems. You'd have to prevent people from inflating their total reputation by using a single site with an easy-to-game reputation system. Some interesting challenges in there :)