I actually became a brony through YouTube
I was surfing YouTube one night, looking for some decent electronic music to listen to.
Then I came across Epic Wub Time: Musicians of Ponyvile. I was skeptical at first, but I gave it a watch.
After that, I searched the internet for the first episode of MLP:FiM (After finding out what the animation was based off)...
and it all progressed from there.
Can't remember the exact date for when I became a brony, though. It was a long while ago!