Or let them register from facebook only while new registrations are closed. I'd also hide any forums to "unregistered/not logged in" usergroup and then in the default registered usergroups hide all the forums and content until you want to officially open the site.
http://www.vbulletin.com/forum/forum...acebookconnect
This will also allow you to heavily publicize the launch of the site via facebook as well which certainly won't hurt a thing

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