A large number of Bitcointalk "users" are just abandoned Brand-New Accounts (where they have to pay to get their Account unlocked because they registered from an evil IP) and even most Newbie accounts are only used for shill posts or paid bumping.
So, the number of real users is much lower but Bitcointalk is most likely still the far biggest forum, not only because it's existing since the beginning and the original but also because the most interesting content is published here. And as far as I've seen the other forums are just like the entire Altcoin Section here on Bitcointalk, flooded by shitcoin spam et al.

Here are some interesting stats, out of 2 Million Bitcointalk users 1,400,000 users are Brand-New (never made a post) and 400,000 are Newbies (most likely only used for paid bumping etc.). Out of 2 Million users only 140,000 users are Jr. Member or above:

2 MILLION users and their STATS