We found that Angie’s List was useful, until it became popular. Then too many people started using it and less reputable people started gaming the system, and then it all went down the toilet.

Same with Craigslist before that.

I would say that the same sort of problem will occur with all these services, unless you can bring new functionality to the problem where you can guarantee the level and quality and speed of service.

See also “tragedy of the commons”.