Forums on ForumMatrix are usually sorted by popularity. Of course it's nearly impossible to determine how popular a Forum really is.
For ForumMatrix we use an indirect measurement. The more popular a Forum is, the more people will write about it on the Internet. And with writing come links…
We use the URL-Metrics API by moz.com to query the approximate number of unique domains linking back to the Forum's home page.
This is not an exact science, but it should be good enough to get a rough idea on how Forums compare in popularity.