This is suggested by Amol.  Combine social nets with GPS location info
to find an any time friends near by, track your friends, etc.  
An aspect of this is to do MobiShare with Social Nets.  So you can
offer a web camera attached to a device and send real images to a friend
in a different location.
One aspect of this could ignore the issue of privacy.  Another may try to
answer these queries at no loss of privacy or partial privacy.