Over the years it's grown to become quite satisfying.
Mmm.. dopamine.
[0] https://thebrag.com/truth-facebook-pull-refresh-feature/
[1] https://www.vice.com/en_us/article/vv5jkb/the-secret-ways-so...
[2] http://www.businessinsider.com/ex-googler-slams-designers-fo...
There is no dark pattern here; yes social media itself is made to be addicting. Pull to refresh is an intuitive UX pattern that can either assist in enabling the addictive behavior, but it is not the cause of it, not only to be used in that kind of scenario.
So given that an explicit refresh button either takes up screen space or is another action (say full page refresh?) should this really be considered a dark pattern if you're say, in your own inbox?
Yes, it's a tool that can be abused in the right (or more aptly, wrong, I guess) circumstances, that doesn't make it a dark pattern in other situations.