Hi!

I'd like to second the WordPress suggestions, and add Drupal on the list as well as a viable alternative. They are both easy platforms to work with.

A better alternative is to find someone that can code it from scratch. My feeling is that this type of service is very hard to differentiate, since there are so many solutions out there, and you'll need a lot of flexibility, which only a pure custom solution will deliver.

A hybrid approach may work best: build something fast with the help of a seasoned WordPress developer, try to get some traction, and then switch to a custom solution once you're confident you can grow the service.

Hope that helps!
Vlad