It depends. If its a very interesting book and I'm anxious to see what happens next, it will keep me awake wanting to read it. However, if its something that isn't exactly thrilling me, I may start drifting off. Same with social media for me. If I'm scrolling through boring posts, I get tired. But if I stumble upon some drama or I see that someone has replied to a comment I made on something with a crappy response and it pisses me off, I'm suddenly awake and arguing. So I tend to be very careful with what I allow myself to read or browse if I want to sleep. I have severe insomnia though, so its not all the fault of books/social media.

Also, with Kindles, at least if its solely an e-reader, they don't have blue light like phones do, which can disrupt sleep. I know there are certain settings on your phone to help with that too but idk if everyone knows that.