So, by now too many of you have seen the growing
Quechup spam.
Like some other social networking services, Quechup allows you to check to see if anyone else you know might use Quechup. It does not do this through some well designed contextualized search such as linkedin, rather, by inviting you to enter webmail logon/password information, and let it check those addresses against the database. What they don't advertise well, is that by default it will also mail everyone in your contact list, inviting them. If you get an invite, please avoid accepting it. If you must accept it, please make sure not to use this option, and warn anyone you wish to invite to avoid the option.
At best it's an innocent piece of badly designed social networking. It'd also be a hell of an effective and low maintenance way to collect active email addresses to sell for marketing purposes.