When XIV started, I remember reading a long time ago a lot of the hardcore elitist (and sometimes douchebag) communities were going to Lindblum. SomethingAwful, 4chan, Bluegartr, some community of griefers from WoW, etc. Lindblum at one point was the most heavily populated server, and I remember during the beta getting tons of RMT spam/shouts/tells in there. I don't know how it is how, but there may be remnants of those communities (after the sharp population drop during October) still there, so I think DeadRiser's statement kinda matches up with the impression I got while mucking about in Lindblum.

From what I've observed though, Besaid seems to have a very stable population. Not necessarily the highest, but a solid number of players you can count on for games.