Yes, no, kinda.

(I am basing all this on these stats: https://lemmy.fediverse.observer/dailystats)

First, the most detailed statistics show "Active Users Monthly". That means, if you have any interaction (e.g. posting a comment) you will be counted as active for a whole month.

If you have a look when the decline first started, you'll see that it's right around one month after the Reddit blackout.

So what happened is that tons of people came to Lemmy during the blackout, tried it out for a few minutes, maybe posted a comment, and then dropped it again. They were still counted in the statistics until the 12th of July, which is when the drop starts in the statistics, because all these "single-day-users" are dropping out.

But: the drop from the highest point to now is only ~10% of the users. Other than that the user count seems to be kinda stable.

For more up-to-date numbers look at the post/comment counts, since they are daily. Here you see a linear, maybe slightly more than that, increase, which indicates a steady amount of interaction.

Btw, the number of total users is steadily decreasing, and that's a good thing. The reason for that is that there are lots of obscure instances with a handful of active users but 10k-90k of users who have never posted anything. These instances usually have open registration without captcha, so all these users are probably bots.

Since these instances don't actually have real users or content, they probably were just created by someone to try something out, so they keep getting closed, and with them, the bot accounts disappear.