Interesting! I've never thought about it like this but I agree. It's not the smartphone but rather the social interaction/media apps on them.
We have a desire for attention from others, and to pay attention to others. We want to watch and see what is going on others lives, but we also want to be seen and monitored for others to pay attention to our own lives.
I think this perpetuates the addiction. It's like a combination of envy and peer pressure. On social media, you only see the nice bits that others share. That in turn makes you feel like your life is not as exciting while at the same time you use social media to convince others (and I guess yourself) that your life, in fact, is. And then it just ends up as a vicious cycle for everyone.