Lets say if it spreads from single member to friends with bubble effect; then having name of the member(preferably genuine name otherwise userid/handle) in subject line can help in long way. e.g. morbidkk wants to keep up with you on Twitter

I personally dont even read invitation mails from strangers.

1) so avoid generic subject line. - A friend invites you to xyz 2) Also check shelfari invite mail; which is very impressive as it just introuduces everything what can be done on shelfari in small but concise mail.