Back near the dawn of time, FB's feed was chronological, and that meant that Every.Single.Farmville.Notification showed up in your feed (and there were a lot of Farmville notifications).
This was actually causing real problems for Facebook, and they introduced an algorithmic feed to fix it. Back then, it was actually relatively simple, and mostly just ranked posts based on how connected you were to the person on Facebook.
Obviously things have changed since then, and algorithmic feeds have had a host of weird impacts, but a ranked feed is not necessarily bad in and of itself.
The mentioned "For You" feed for BlueSky appears to be a variation of LinkLonk's algorithm (by the same author), so how it works is partially known: https://linklonk.com/item/3292763817660940288
I kinda wish I could influence my For You with rules:
> No more than 2 degrees of separation from me
> Posts with fewer than 150 likes