Indeed. I took out all of the default subreddits because they added too much noise - everyone starts out subscribed to all of them.

Can you not just flip the switch and get equally useful data - don't look for people who subscribe to a default, but to those who unsubscribe from it?

IIRC that can be hard to tell since you can't get the subreddits that people are subscribed to, just the ones they comment on, or post in. Many people are subscribed to r/wtf but don't post or comment in it.