That it's so employees will quit is the truth of the matter. We shouldn't assume that this is to target "dead wood underperformers". Facebook has a ridiculous performance review scheme that, whatever else it does, is effective at firing people.
They already push people out to sea as a matter of course -- what's happening now is that the economic outlook is unfavorable, and they need people, including decent employees, to quit. They declared a hiring freeze and have announced a policy of attrition. These are normal things that happen during a downturn. It shouldn't be framed as somehow making their workforce more excellent, or as if the people who'll lose their job were "dead wood" anyway. The economy is going to put a lot of people out of work this year, of all sorts.