Private email could work, but there are not built in tools to help the person process the feedback once they have received it. People receiving feedback have the option of 1. ignoring/deleting; 2. asking for more feedback from the giver; 3. asking their friends via email if it is accurate; or 4. asking people via facebook/twitter if it is accurate.

Also, most people do not talk frankly with others. They just let the person continue to make mistakes or let them off the hook. That is part of the problem we are trying to solve.

A lot of people can't discuss things in a very matter of fact nature.