Want to invite your email contacts to join you on your favorite social networking site? You probably can. Want to do so without having to give your email account username and password to the social networking site? Your odds drop a bit. Want to do it all in a standardized way? Your odds drop to zero.
But that could change, if the guys behind the Portable Contacts effort have their way. They're trying to provide a standardized and secure way for users to share address book data across different sites.
The specification is still at the draft stage. But, as Dare Obasanjo describes, there is already a lot to like about its approach (particularly the alignment with OAuth and OpenSocial). So while it's too early for a social networking server like Project SocialSite to directly leverage this work, we'll certainly be keeping an eye on it.