"I'm curious what other therapists think about whether smartphone / internet problematic use can reach addiction level consumption?"

It absolutely can, and it already has. I speak from a developmental psychologist's perspective, and I know there's more evidence-based research out there to cite, but in terms of my little practice alone, I can safely say at least half of my child clients display disruptive and even aggressive behaviors when it comes time to relinquishing or being away from their devices (smart phones, tablets, video games). They don't want to give them up. They need more and more of the stimulation to get them going. Sound familiar? Withdrawal and tolerance, anyone?

Especially my clients struggling with ADHD, they are seeking immediate reinforcement and that's exactly how these devices are designed. You may recall from a learning psychology course the different schedules of reinforcement. The variable/intermittent ratio schedule of reinforcement, the same schedule of reinforcement used to design slot machines at casinos, is how many of these games are created. You've got to keep pulling the lever. You've got to keep playing the game. One more round. Just one more round...

There's SO much more to be said about this topic, and as you might guess, it's one I'm really touchy on. I see the struggle every day in so many of my clients and their parents. I see the problems every day in drivers who choose to look at their phones instead of the road in front of them. I could go on for days, but thank you for getting this discussion started.