For me it's about actively engaging with other fans (for me it was on LiveJournal), generating new content, keeping up with news, etc.

For instance, ten years ago I was very involved in the Harry Potter fandom. I was a member of several roleplays, wrote fanfiction, read The Leaky Cauldron religiously. But that's not something I have kept up with, so I don't consider myself to be *in* the fandom right now.