Probably monthly active uses would be the best gauge. It's somewhat over 50k for Lemmy compared to around a half billion for reddit. That would be .01% or one ten thousandth. So even if all those were the result of people leaving Reddit, a graph on paper would not have high enough resolution to register a difference.

In any case it only matters that Lemmy has a big enough user base to make it worthwhile. I'd be more concerned about it getting too big than being too small.