Avatars United ( www.avatarsunited.com ) was initiated in 2007 by a group of childhood friends sharing a common passion for immersive online worlds. The idea behind the community is to help people keep track of their in-game friends, old and new. Great friendships are tied within virtual spaces yet we felt there was a lack of a single tool to help keep track and share experiences with them. As such, Avatars United is a free community website bringing avatars from all online worlds together.
I would say that the fundamental difference between AU and other communities is that AU is a community for avatars of all online worlds. As a user you can register multiple avatars in multiple worlds. If one of your friends would migrate from say vSide to IMVU, AU would help you keep track of him/her and share of all of his/her new adventures. As your friend would still keep his/her vSide profile registered anyone wanting to track him/her down would find it easy doing so.