Having been at FB as well as multiple startups that became unicorns, I can tell you the "high stress" at FB is nothing compared to startups. On a whole, FB is a great place to work with very solid WLB.

My theory on the FB comments is because a lot of employees at FB have not worked elsewhere and compare to friends at Google or Microsoft. If you compare FB to startups or even finance, you're working significantly less with less aggressive targets and pressure.