I'm at a loss as to how this is surprising anyone. How did people think that these apps found other users you know? This is built to support: A) finding existing people on the service and B) so they can (theoretically) send you notifications if a friend joins. If you want those features (and it seems that users do), this is the only way to do it. Admittedly, most apps are more explicit about it with a "find friends from address book," but if you want to lower the friction as much as possible, this is the way to do it.