Every time they get rid of the most egregious hosts and replace them with new ones. The new ones start out moderate, then inevitably drift to extremism. The advertisers have plausible deniability for half of their employees and customers. The other half don't care or are cheering Fox.
Mutual fund companies do something similar with underperforming funds. They cull the worst performers and the remainder make the whole fund family look good due to the magic of survivorship bias.
Facebook is obviously drastically different than Fox. That said, they are a large company and organizational inertia is very hard to overcome. Right now FB is optimized to promote quantity of engagement without regard to quality. Turning down the heat and promoting civility would result in fewer posts, which would mean fewer ad impressions. That's not happening.