There’s a new mobile sns company in Shanhai called Belinker. The idea is to link people with geographic information via SMS. Here’s how it works: You send your location to the service via SMS. Then your friends will be informed by the service about your location. Probably only friends near the same location should be informed. But this is based on if they have reported their location to the service beforehand. I don’t know if this works, since many people actually doesn’t want to be traced.