I don't believe this game is popular at all to be honest. In fact I know for a fact that activity has gone down, and users per year have gone down from what I can see. I am on at different times throughout the day, and nothing changes, the number of users remains to be rather small.
When I first joined somewhere in '09 I remember rooms because full and lots of fun energetic people, but now the popularity has gone down. I know that many play single player, but if this game was truly popular there would still be an abundance of people playing multiplayer.
And even on the forums, I was used to see 400-500 users online at once, but now the number drops to the low 150-300 users online at once.