Well just on the forums alone
http://www.firefallthegame.com/community/members/
26850 pages x 20 members per page = 537000 members who actively use the forums.
i cant find any server status information yet but the games still in beta, however when i play around 6 pm GMT it gets very laggy with he sheer load of players.
They are spending alot of money on
PR and advertising and i often see banners on most of the major websites i use such as youtube.
if they this popular in open Beta, i assume their popularity will boom once open.
This game is set to be free to play, upon release.