I didn't fully quit reddit, but I'm going to Lemmy first and foremost and rarely go back to reddit for very specific communities. My reddit usage dropped by 90+% probably, but I'm not completely gone.
I'm sure the same is true for many other users as well, so simply counting the number of (active) users then and now won't get even close to the actual loss in traffic and participation.
I haven't gone back to Reddit yet, but I'm sure I will at sometime for, like you said, some of the smaller communities or something specific I may not be able to find here for now. I'll stay here as much as I can though, I'm sure it will only improve.