Quechup isn't the first SNS to use this scheme for increasing its userbase. Tagged.com is another site that caused frustrations to echo across blogs. They basically did/do the same thing.

A previous commenter mentioned Ning, I imagine they do something similar.

Facebook offers the same 'lend us your webmail login details and we'll search your address book' to search for new users.

Certainly there are arguments in favor of making the process easier, but if username/password are the only credentials you have to verify that you are the owner of X account on Y webservice, it strikes me that this is at best an unsafe practice that is quickly gaining ground.