I just joined this March. My guess is that the most popular month will either be July or August. Since I've noticed a lot of accounts will post a lot to begin with and then abandon the site after a few months. Therefore a lot of people will have joined recently, with less older accounts to balance out the newcomers. That's just my guess though, and it could honestly end up being anything.
I joined in July of 2013, the year ACNL was released. I don’t remember exactly, but I think I joined a month or a couple months after it was released. I have the vague memory of browsing the site as just a guest for a bit and then joining.
Man, that was so long ago though. I cannot believe I was only 16 then and I’m 21 now. I’m still young, but I’ve never really liked growing older. It happens to us all though.
I joined December 2017 and then didn't touch the site. It was mid-July 2018 when I started using it averagely then when August hit I was obsessed with it.