Was there even a mass exodus? I largely avoid Reddit now, but I do kind of doubt that they've been hurt in any meaningful way by all the protests and people leaving...
Migration goes beyond sheer numbers. The 3.8k users are probably the one that were the most attached to initial Reddit, hence people who would contribute the more. I would rather be with those 3.8k users than the millions of people okay with staying on Reddit despite Spez's decisions.
I hope that once Lemmy is a bit more polished (instance blocking, account migration, hot filtering working etc.), we will gradually see a second wave of arrivals.