In most big companies I've worked at, I thought I could identify the people who made everyone slower and were a net negative to productivity. However, they never seemed to be the ones threatened to be fired. Often they were really good at playing corporate games so that it was really hard to justify firing them, while the ones fired were junior employees who had great potential but weren't as good at politics yet.

So while FB certainly intended to fire underperformers and increase average performance, I doubt they were able to identify the right people for that.