Well, Social media and smart phones might not decrease interpersonal contact, I think it does change the way we communicate with each other when we meet. Before social media and smart phones, everyone would hold a conversation and have what to talk about.
Now? You meet up with people and they start scrolling through facebook feed (or whatever else), without even thinking of having conversations with the people around them.
Social media is like any other tool, it can be harmful or useful depending how you use it. For example, take a hammer, it's a useful tool right? You can fix things with it, but you can easily take the same hammer and smash someone's head, but does that make the tool bad? It's all about how we use our tools.
In my opinion, most people use social medias the wrong way, they overuse it and thus on some level it affects the way we communicate.