From my perspective, someone who bought the Marshall Mathers MP when it came and loved the song Stan (I'm mid 30s). I would say "Stanning" isn't ubiquitous at all, in fact, I've never heard of it until today.
Just did a quick check around my office (Fintech company)(mostly 25-35) and no one had heard of it.
So yeah, I don't think it's that commonly used.