Social media and other addictive Internet sites - people are constantly glued to them, even on their work, which decreases their productivity. Social media also cause depression in many people, because they not only procrastinate, but also constantly compare their life to others, which leads to dissatisfaction. Also social media are good at spreading lies and fakes, which can have negative effects on the economy - anti-vax panic, anti-GMO, political manipulation and so on.
I agree, young adults now are very addicted to social media that they don't care so much about study but they wanted to have their profile to have a lot of subscribers. Indeed social media can make a person to be unproductive in the sense that because of Facebook for example they will not do their work with quality. Also not all in social media can be trusted some new can be fake which can manipulate or stress people.