Perhaps a contributing factor is how HN shows only the final non-eTLD [0] label of the domain. If it showed all labels, you'd have seen "engineering.fb.com" which, while not a dead giveaway, implies that the problem space is technical.
It would be nice if this aggressive truncation were applied only above a certain threshold of length.
We are actually saying different things, and your point highlights an error in mine (i.e., I assumed they show the eTLD from the PSL plus one extra label, but apparently they have their own shadow PSL which omits things like pp.se and therefore occasionally shows nothing but an eTLD?) but either way we agree that showing more would be better.
Seeing the title and the domain I thought this was user profiling and I was wondering why would Meta be publishing this.