Loopt Mobile Messenger, Friend Finder, and Event Service
http://www.loopt.com/, description via Joe McCarthy
...they already have 40K users, and will be doing a
major launch in December (at which point the application
will be pre-installed on all Boost phones).
...Managing privacy issues will be crucial to their success;
Mark said everything is opt-in, requiring an explicit
invitation, acceptance, and activation, and it only works
if you know someone's phone number."