This is odd.
Exchange Admin Centre in O365 lists Facebook Contact Sync as being enabled under the Default OWAMailbox Policy but when you edit the policy there is no Facebook Contact Sync option listed.
MS Support say that Facebook sync is no longer available - which is born out by a quick search:
But that's referring to Facebook Connect from 2013! Is that the same thing?
Confirming that with MS Support.
They have said you need to run a Powershell command to check settings:
Set-OwaMailboxPolicy -Identity <Name of the Policy> FaceBookEnabled $False