The population on servers I know of (Fenrir and Asura) tends to be around 600 at the American regular play-times, but numbers well over 1600 can be seen during the European/Japanese times.
The game has seen the implementation of NPC companions going by the name of Faith/Trust magic; alter egos who are essentially copies of famous NPCs such as Tenzen and Nashmeira.
They're somewhat similar to adventuring fellows, yet different, falling short in some areas such as not having a soul at all, and more, but they're still a relatively new thing. These NPCs allow people to create their own parties, so to speak, and actually level up even without Abyssea.
Speaking of Abyssea, some people still 'leech' their job levels up there, so that works too.
I generally play with a friend or few, and don't even have Seekers of Adōulin, but I still like hanging around I guess. I may not be the best person to say if it's worth playing, but I'd definitely say it's worth trying out for one to see for themselves!
Just some quick-like thoughts!
~ The Noob Unlimited ~
Sore wa sore, kore wa kore.