Facebook user agent will be one of these:
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)
facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)
facebookexternalhit/1.1 (+https://www.facebook.com/externalhit_uatext.php)
facebookexternalhit/1.0 (+https://www.facebook.com/externalhit_uatext.php)
Look for these in request.META['HTTP_USER_AGENT']
and redirect.