Apparently Facebook sends out notifications when a friend who hasn't posted in awhile does post. I don't know if this signal manifests itself outside of notifications, i.e. whether these first-post-in-awhile get high placement in a "Top Stories" newsfeed.

https://www.reddit.com/r/facebook/comments/6u5twl/friend_pos...

I know that as a sporadic user myself, I get mobile notifications telling me things like it's been "6 weeks since you've last posted", or that it's about time I change my profile pic.