Just a thought from my own experience. I was always anxious and was diagnosed with an anxiety disorder way before I got an iPhone and I realized that I need to cut back on the scrolling. I also notice that reading something and always having/doing something with my hands has always been a big part of how I calm down and relieve my anxiety. So, I think that scrolling on Facebook or my phone is my new way of coping with anxiety or depression in the same way people smoke cigarettes when they're stressed out. I don't think it's the phone/Facebook that's makes me anxious, but the other way around. 

Anyone else can relate or see any truth to that? I try to look for articles on this, but it seems that every articles simples says the same thing about how screens cause mental health issues and not the other way around.