From wikipedia, emphasis my own: "Six degrees of separation is the theory that everyone and everything is six or fewer steps away, by way of introduction, from any other person in the world".
This does not mean that any two randomly selected people will, on average, be separated by six degrees. It is the (theorized) maximum of the separation between all possible combinations of two people on Earth.
Facebook calculated a different metric: the average separation. And at this, they only calculate it among Facebook's own users - not all living people. Yet they present their findings as if they are directly applicable to the initial problem statement, which they aren't.
I mean, some groups of people on earth are uncontactable or their existence is unknown (see Sentinelese, people in the Amazon). So you have to make a compromise at some point. The choice of percentile is somewhat arbitrary.
Alternatively, you could limit your population of interest to the largest group of people that can all find a route to each other. In practice, this just boils down to excluding "uncontacted/unknown" peoples.
It's a bit different than choosing a percentile to target, and also a bit less arbitrary, in my opinion.
If they're going to be referencing the six degrees of separation theory, what they should really be doing is making the same assumptions that were used in that theory (whatever those were).
Are they deliberately lying, or incompetent?
6 degrees hasn't shrunk. Facebook is merely counting something else. Why doesn't anyone point this out? I feel like I'm taking crazy pills.