SELECT COUNT(*)
FROM xf_user_external_auth AS ext
INNER JOIN xf_user AS user ON
(user.user_id = ext.user_id)
WHERE ext.provider = 'facebook'
AND user.register_date > unix_timestamp('2014-01-01')
That will give you the number of Facebook registrations since 1st January 2014 (YYYY-MM-DD).
Change the date accordingly (or remove that line to count them all).