Some small instances are seeing sudden increases in numbers of registered users (4k to 5k new users) but not an equivalent growth in activity, my guess is someone is creating accounts on instances without captcha enabled for account registration. Most of the top 10 fastest growing instances shown here fit that criteria.
So going to that instance, and going to its list of federated communities (https://parapheum.com/communities/listing_type/All/page/1) shows a few sizeable ones. Also note the instance's description (see https://parapheum.com/) which is basically to distribute the server load without any commitment to any particular kind of community.
So, could be full of lurkers, or parallel accounts created to avoid server overload that will be soon dropped. So probably some bloat in these numbers, as there are for other platforms too
Looking at their numbers on their home page, it looks like people might be moving off of the instance, but again, people have definitely subscribed to various communites there and might just be lurking.
Here's an insightful comment I ran into, it looks like lemmy.podycust.co.uk / parapheum users don't seem to be making much interactions with other instances.