That may sound like not a lot, but Facebook as been hemorraging users for a few years now, if they're losing users at about the same rate as Facebook, that's a big oof.
I think the big deal will be if it's sustained. Losing a bunch of users for a month isn't a big deal if they come back, or at least stop leaving. If Reddit loses 3% of its users every month for a year then things will be pretty dire for them.
Can't say I've got much sympathy for Reddit, though.
I would expect July to be higher since 3rd party apps were still functioning in June. That was the first wave, the second wave would have been after the apps actually shut down and will continue for a while as people see lower quality and people talking about other sites.
Another major element is the effectiveness of moderating with the lack of API tools. If that 3% consists largely of moderators and content creators then that is going to have an impact.
The thing that kept me on reddit were the well managed niche subs. I really need to take the time to go through my pages and pages of subscriptions, and either find or start equivalent subs here.
I rarely commented on reddit, and almost never posted. The one sub I created never had a single use. Here though, I feel almost compelled to contribute as it is new and small and I want it to grow.
When the 2nd and 3rd waves of users come looking at Lemmy to see if there is an alternative, I think there should be quality subs and content to get them to stay. It's on us to provide that.
Facebook "lost" a lot of users when GenZ decided they didn't want to make accounts, but Instagram and (likely) Threads, did a fine job supplementing that. Meta corporation as a whole doesn't have a big issue with maintaining their userbase.