Fox News is upset that Twitter didn't more pro-actively protect Tucker Carlson when he was attacked (although the police report doesn't match up with what Tucker has been claiming). So, they are currently refusing to Tweet and instructing their people not to Tweet. TV Newser has the details. It's an informal protest.
I personally think it can't be effective unless they do it permanently. Seriously. No other way to make your point. Silence. Please. Forever. That'll show 'em.