Number of unique /b/ users?

I noticed that Alexa puts 4chan in the top 2000 sites online.  That implies that there must be thousands of 4chan users. (I would assume).  I have also read that it takes /b/ only 23 days to produce a million posts.

So, do we have an estimate for the number of /b/ users?  Essentially, I ask, is Anonymous truly legion?

Habbo raids include other sites though, like myg0t and a bunch of bantown.