Originally Posted by Emivader
I've already linked Facebook and Twitter but that doesn't seem to change anything
They meant your Xbox's privacy settings, not GTA V's.
From Xbox dashboard: settings/privacy/change settings/customize and see if Social Network Sharing is set to allowed
EDIT: From your Rockstar social page I checked your Facebook and twitter feeds and the game hasn't posted a single message on either, so I think there is a good chance it is your XBOX's privacy settings.