The real reason doesn't matter. If a company leaves and says it's due to Facebook's promotion of hate speech then there's no way that company can go back to advertising on Facebook before that changes (it'd look like a change of heart, and that now they think promoting hate speech is OK). Facebook are seeing a drop in ad revenue no matter what, and the only way to get the companies back will be to change. Maybe FB don't care about getting them back, but I suspect that's not likely.