Other components such as the search were probably also quite tricky. One thing i've never figured out is how Facebook handles search on your timeline. That a seriously complex problem.
Linkedin Recently published a bunch of papers about their feed tech:
https://engineering.linkedin.com/blog/2016/03/followfeed--li...
And Stream's stackshare is also interesting: https://stackshare.io/stream/stream-and-go-news-feeds-for-ov...