If the user already has a Facebook account, the invites will be delivered to them via a Facebook message only. For those that do not have FB accounts, they should go out by email.
Facebook changed their Invite logic recently, which is causing the error page your seeing. We're still trying to diagnose the cause, as it's not happening to everyone. One poster, linked below, described how he fixed it. Please verify you have your Inviter and Invitee URLs setup. For us, we simply use our home page address:
www.sourcecoast.com/forums/topic?id=1144&p=1#p4168 
Invite feature problem: sends invites but not via email
