I think part of the reason people (especially younger audiences) are using Instagram/Twitter/Snapchat for key functionality over what FB provides is because of the simple UI/UXs. Kids might be seeing FB similar to how they see Excel spreadsheets. Complicated shit for adults that isn't synonymous with "fun".
You could also argue that Facebook focuses too much on content consumption whereas Twitter, Instagram, and Snapchat all focus much more on content creation.
Another problem I've seen a lot lately is the Newsfeed algorithm just being downright terrible. I've noticed a rash of comments on months-old photos lately where people are saying "how did I miss this photo?!" If you're going out of your way to create content and no one sees it, what's the point?
The competition for a spot in the users' news feed makes it increasingly harder to present the user with quality content...I sometimes wonder how it's even possible to test the quality of their algorithms a such a large scale (kudos, btw), even with all that automated continuous learning.
Do you remember when you had all those wall plugins like drawing apps, and "SUPER POKE" it got way out of hand.
And part of the reason Facebook did so well when it first came out, was because it was a far easier alternative than the CSS bloat that was on Myspace.
Remember going to your friends page, and three videos would autoload, with 7 songs playing and glitter graphics?