An individual known only through a social network. Someone whose wall you will post on and share from, but won't likely start an actual chat with because you don't know them well enough.
"Are you inviting Bellis to the party?"
"Who the @$%# is Bellis?"
"That guy whose lolcat pic you shared on your wall!"
"Oh, no, no way. That guy is just a Facebooquaintance."