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One controversial issue among social networks is how hard they should push for user acquisition. Most social networks these days let you to import your email address book in some way (Twitter is the latest), but most make it clear if they're about to mail your contacts.
One site that's catching people off guard is Quechup: we've got a volley of complaints about them in the mailbox this weekend, and a quick Google reveals that others were caught out too.
The issue lies with their "check for friends" form: during signup you're asked to enter your email address and password to see whether any of your friends are already on the service. Enter the password, however, and it will proceed to mail all your contacts without asking permission. This has led to many users issuing apologies to their friends for "spamming" them inadvertently. Hopefully the bad PR on this one will force them to change the system.