I was a Software Engineering intern on an ads product team at Facebook last summer. My experience was almost exactly the opposite of what you assume here. Most people worked 7-8 hour days. I stayed plenty of late nights because I was dead-set on getting a full-time offer, but I was literally the only one on my entire floor most nights (including interns). On the off chance someone stopped in late and saw me, they were honestly shocked and insisted I go home.
The team working next to mine had a major product launch during the time I was there. A few of them (2 or 3) stayed in until 8 PM for the two nights immediate proceeding launch. That's it.
One of the biggest surprises to me coming from undergrad was just how little the engineers worked at a company like FB. There is a pride in efficiency in the Facebook culture, and an working effectively is more valued than putting in long hours.
Of course, that was my experience on one team. I'm sure some teams work longer hours than others. That said, I think you'd be hard pressed to find an engineering shop that had a healthier work-life balance than Facebook. The idea that its a 'meat grinder' is laughable. I saw another comment suggesting that FB employees leaving after 8-10 years is a sign of burn-out. Are you kidding? In this business 8-10 years is lifetimes.