Like, actually getting into the fandom and/or watching the show would be my definition of it. I saw this quite a while before I ever really participated in the fandom, although I was in retrospect already well on my way to becoming a brony. The music is what started it. A small, seemingly insignificant decision of what song to play. Which resulted in me discovering the amazing work of All Levels at Once. Leading tangentially to the discovery of Aviators, and a plethora of other amazing brony music artists. Oh, and also this one guy I know on Steam who kind of convinced me to watch the show, but I don't think I would have if the music hadn't been a part of the decision.
That was September of 2013. I most certainly consider myself a brony, am actively involved in pony fanfiction (reading and writing, as well as proofreading and editing), and also participate in a few other fandom-related activities, including roleplaying. I've made lots of friends since I joined the fandom, and not just other bronies. It's helped me to become a more outgoing and accepting person, and the people I've met have taught me so much. I'm proud and grateful to be part of this fandom and to know so many amazing people.