What this doesn't show is active users, just total. For quite some time we've been one of the most active large instances. I don't remember the exact timing, but we've held the spot of 3rd most active for some time, we just now have total users to match that.
In short, engagement. A bunch of us migrated off another Reddit alternative which promised a lot of what we set out to do, but in our opinion failed in moderating itself and actually promoting a nice environment. This core group of individuals has been enthusiastic about carving a little space for ourselves on the internet for like-minded people and that dedication that comes alongside engagement is what kept us active and slowly growing.