I can say what has reduced my Facebook use, if anybody in a relevant position at the company is reading. It's the content Facebook shows me.
A decade or so ago, most of the content I saw on Facebook was original content from people I know. Most posts were either text written by someone I knew, or a photo taken by someone I knew. Perhaps not everybody shares this position, but what I most want from Facebook is to keep up with people I actually know. Here are the first 20 posts I see now:
7 shared images from strangers/pages. 1 screenshot from a TV show uploaded by a friend. 1 page updated its website. 1 shared video from strangers/pages. 2 "suggested" videos from a page. 1 photograph taken by a friend. 1 "memory" containing text written by the person who shared it. 3 links to a news story. 3 posts by a page.
That's 2/20 posts of original content from people I know. I've noticed that if I post original content, my friends are less likely to interact with it than a few years ago, probably because they, too are presented with such a huge amount of other content.
I know I can opt in to something like the old experience with the "friends feed" feature. Well, I can on the desktop site (if I bookmark it, click it each time, or use a browser extension); it appears to be missing from the mobile site and the Facebook Lite app. Anything I post is still disadvantaged though, because most people are using the default algorithmic feed.
On Instagram, every one of the first 20 posts I see is original content by a person or page I follow. 16 of those are individual people. I see shared third-party content on Instagram fairly rarely, probably because its UI doesn't encourage that.
What would get me to use Facebook more actively is a change to the algorithm that advantages original content over shared third-party content.