I think it’s just the nature of the platforms. On mastodon and Pixelfed you mostly come across users that you already follow, and the content isn’t geared towards replying. Whereas on Lemmy, you can get content from many users and that content is geared towards commenting

Too lazy to find the exact date, but at some point devs changed how they report user activity, and old instances were reporting incorrectly until they upgraded. The dip might align with that change?

We do not speak of it!