NA/EU population has declined dramatically on Bahamut in the last few months. That could be people doing summer things, a mass of people afking from the game due to lack of content, but I believe most of this is server transfers to Asura.
EU community is non-existent now, and there really are only 2-3 LS capable of doing anything T4 with moderate or total success.
We are completely full and don't really take people unless they well known by current members, (A lot of our people currently aren't playing).
Not sure if the other 'top' Ls is taking people (Blame Mochi), my understanding is their 'recruitment process' is like ours, and Empyrean has been attempting to transition from social to more than that. (This LS might be your best bet)
There are a hand full of other socials around.
But like Elazar said, we went from around 600 to 300 during NA/EU times on weekdays, and during Friday/SAT/SUN it's about 500-700 i'd say with closest to 600.
This is a high JP population server, and that can get up to 1.2k on the weekends.
Tl;dr People who want end game and don't get into Inquisition or BlameMochi, end up leveling SMN and hoping to Asura, or just buy clears/aeonics. If the end game of 1.5 years ago isn't a concern for you, come to Bahamut.