I don’t tell anyone about it, also a lot of my imageboard usage was during a part of my life when I was depressed/antisocial so I prefer to leave it in the past where it belongs.
My sister used r9k and she knows I’ve used 4chan, but that’s the extent of how much we’ve talked about imageboards.
I don’t plan on telling anybody I meet unless it becomes relevant in a conversation, to most people imageboards are spaces for rejects, bigots, and losers and it rarely helps to tell someone that you were in those spaces unless they have a more nuanced understanding of imageboard culture.