With yesterday’s addition of AT&T, Loopt can boast interoperability for its location service on every major U.S. carrier.

Not surprisingly, more than 50 percent of Loopt’s friend invites are sent between different phones and networks, so having interoperability between operators is key.

A Loopt representative says AT&T will support its launch with a nationwide marketing campaign that will include in-store displays, online advertising and youth marketing activities.

Loopt also says is has surpassed the 1 million registered user mark for its social mapping service.