I just checked a few Traditional Catholic fora to see how busy they all were at a given moment in time.
There are many metrics you could use (# of posts, # of members, # of guests/members, etc.) but for this test, I wanted to look only at the number of registered members logged in.
Here is a complete list of all "Traditional Catholic" fora:
Forum name: number of users online when I checked (number of members total, how long forum has existed)
CathInfo: 24 members (3,200 members/5,194 before trimming, since 2006)
Suscipe Domine: 11 members (1,058 members, since 2012)
Le Forum Catholique (hard to get stats, but this French forum seems decently active)
Fisheaters: 9 members (5,728 members, since 2004)
ABL Forum 3.0: 1 member (54 members, Jan 2016)
TradCath Forum: 1 member (43 members, since Apr 2016)
Te Deum: 0 members (274 members, since Sept 2014)
Cor Mariae: 0 members (140 members, since Aug 2013)
ABL Forum 2.1: 0 members (94 members, since Oct 2015)
Catholic Answers: 82 members (492,604 members, since 2004)
but, of course, that forum is actually anti-Traditional -- so it doesn't count in this list
An interesting note -- some fora have large numbers of members, but very little traffic. That seems to suggest a forum whose best years are behind it, or at least with a very "fluffy" member database. That is to say, a lot of defunct members cluttering up the database.
P.S. The latest member to sign up for CathInfo is member #5,194. That is because a couple years ago I purged the member database to the tune of almost 2,000 defunct/banned members, or those I'd like to forget about. So if I hadn't done that, CathInfo would have "5,194 members".
But gross membership numbers aren't everything. The most important metric for a forum is actual traffic by real humans.