Ha! Like a few others here, I started on Neopets about 10 years ago. I was just clicking through until I ended up on the roleplaying board. Mostly camp, high school, and Harry Potter at that time. To be fair, that was the only thing ever on offer, haha. If anyone tried to set up anything else, they'd end up pretty lonely because the roleplay would end up ignored.
While on Neopets, I found myself really loving Harry Potter so from there I ended up on the Harry Potter forums. Side note to anyone familiar with the Harry Potter forums, I found out a while back that the place STILL exists. The WB website stopped providing links to it, but it remained open. Recently, they have re-added the link to the forum on the site. It looks exactly the same as it did 10 years ago. Sadly, it does seem that they've cleared out a lot of the old posts. It is still nice to look over the board for the sake of nostalgia, though. And you can still find people roleplaying on it!
Anyways, on the Harry Potter boards, I found people linking to offsite Harry Potter roleplay forums on proboards and invisionfree. They were full boards where you'd create a character and that character would basically live their life at Hogwarts. There were even board-wide plots that created an alternate history or followed the book timelines. It was great fun.
From those I went on to different fandoms that had their own dedicated invisionfree boards, and eventually to other boards similar to this one filled with one-shots of every genre. Then I finally ended up here!