Smartphone use is the symptom, not the cause of loneliness. Young people today are forbidden the tools used by past generations to counter loneliness. Ask an urban highschooler how often they just hung out at a friend's house, or in a public place, for more than an hour or two. Teenagers today are busy. They have homework and courses to do after school. They are supervised, chaperoned, by adults far more than previous generations. The smartphone is the place they get to chat with friends out parental earshot.
Even when they have the time, many normal activities are now forbidden. Parents don't leave teenagers home alone on weekends anymore. House parties are unheard of. Teenager-friendly places like arcades, malls, even local movie theaters are disappearing. And the personal transport, cars, that once knitted teenage culture together are disappearing. Cars are expensive, risky, and put kids in more regular contact with police than is safe. So they stay home. All of these things are replaced by the new malls: whatsapp and snapchat.
Teenagers are not being drawn to smartphones, they are fleeing to them,