A 'feed' does not have to be overtly social to have addictive potential.
Suppose you visit a news site, not a social one, no likes, hearts or any feedback from other users.
The stories appear in a feed that you can refresh. 80% of the stories are of no interest to you, but 20% are great.
You can't predict when an interesting story will appear.
This configuration is enough to create "positive reinforcement" and the unpredictability can foster addictive behaviour.
When you encounter an article you like, it's a bit like sifting through sand and finding a gold nugget, you get a dopamine hit for that.
The fact that the nuggets only come up randomly and are hard to predict is called an "intermittent reinforcement schedule" and is known to elicit a high rate of response from the subject.
[1] https://en.wikipedia.org/wiki/Operant_conditioning_chamber#C....
[2] https://en.wikipedia.org/wiki/Reinforcement#Simple_schedules
Maybe you mean kuro5hin?
When I did use Facebook daily, it was for all the reasons you mention. If I ever log onto the site now I'm immediately driven away, it's just a bunch of advertising and nothing I care to see.
Back when it was more about the profile, about "stalking" friends and connecting with real-life acquaintances, it was much more addictive. Features like "you know this person via these two steps", poking, but also subtle clues like the timing of when somebody visited your page, or that they commented on an old photo (meaning they browsed your gallery)... that is all missing now. Facebook was so addictive because it enabled you to obsess about social connections. Friends, crushes, people with common hobbies, and so on.