Look into email addresses or contacts on FB, twitter and suggest invitation.
There may be an aggregated email that may be sent to me that allows me to quickly invite friends from gmail.com, facebook, twitter or other email. I don't want to think about friends' email addresses when I invite them. ;)
Alex Yakunin commented
We're working on tight integration with FB: it will be possible to use your FB friends nearly as 9facts friends. Invitations from FB are already done, so you'll be able to use them right after the nearest update.