One of the earliest things I can remember was encountering a thread on the forums of nuklearpower.com (home of the 8-Bit Theater webcomic) that simply asked, "Religious people, why do you believe in God?" and that was the first time I ever had ever encountered atheist perspectives or questioned what my parents taught me. At the time, there was very much this idea of, "Nobody ever changed their mind from an internet argument" but the internet exposed me to a lot of different views that I would never have encountered otherwise (see also: queer people).
Other than that, I used to gather around with friends to browse icanhazcheezeburger and failblog and stuff. I stayed up late grinding levels in RuneScape. Newgrounds and flash games were a big thing. Some of my friends were into 4chan in the early days when it was more about edgy shock humor than straight up Nazis. There was social media like MySpace and Facebook but I had no interest in them bc I was a nerd. There were a lot more random little websites that passed around by word of mouth.