Speaking for myself, if was a little bigger than 20 members (around 100). The thing is that we wanted to keep the forum open to everyone, at least for the public parts. That's why I went with invisible tweaks (mostly just renaming fields) instead of a challenge.

But I could have came up with a challenge that anyone interested in our forum would have known. We were a rhythm games club, so a questions could have been "We organize a tournament for this game, in 3 letters". Anyone who heard of us would know it is DDR (it was our main event), and for those who didn't, all it took was a quick look at our website.

And even if it is just a hobby, you don't get the same thing from a forum compared to a WhatsApp or even a Facebook group. In fact, a few years after me and all the geeky admins left, they switched to Facebook, which completely killed the already dying online community.

Maybe nowadays we could use a Discord server, which does a bit more for a community than an WhatsApp group, and is more accessible than IRC, and saves history.

Another great thing about the forum is that I still have a backup from the time I still had admin rights, and I could bring it back to life on a private server with some friends, even though the club is now completely dead.

Memories are not lost, except for most of the external links (ImageShack...).