When I was 9 or 10 I decided I wanted to dress up as a character from a show I was watching at the time for carnival.
Together with my mom I performed a web search in order to find images of that character.
We found a website specializing in that show and after a while I found out it had a forum attached to it. My mom allowed me to register there and I started to participate in it.
Most of my social contacts during my teen years were in online communities I found indirectly through that forum.