But does that really require an absolute time reference?

In these cases mostly not, they require low jitter references and a good counter.

An example that does - I work for IceCube, a big neutrino detector at the South Pole. The detector is essentially a time of flight system that detects emitted light as it propagates through the ice when a neutrino interaction happens. We need good time distribution in the system so we can marry up hits from the 5000 odd sensors in the ice (and successive hits may be nanoseconds apart), and we need good absolute precision because we collaborate with other observatories.

Also pretty much all radio telescopes that do long baseline interferometry need excellent absolute timing. The South Pole Telescope has (I think) the most accurate clock on station, a hydrogen maser-based system. We use GPS.

Neat!

I had the same question - there are obviously a lot of applications for this, but why would a company like FB need it?

Distributed databases I assume