Having been burned by the spamming techniques of the social site Quechup last year, this year I was smart enough to ignore invitations that have begun showing up from relative strangers to sign up to another social site Reunion.com.
You'd think they would learn. Tricking people into thinking that they're going to be given a chance to invite people they know only to have the system email every single person who ever sent or received email from them is Bad Business. See my comments on Quechup. See the Wikipedia entry for information as to further bad business practices Reunion has undertaken.