What is Acceptable: Two people using one phone. It's like saying two kids playing with one toy is bad.
What is NOT Acceptable: One person using their phone and ditching the others next to them.
What is Acceptable: Two or more people are using their phones and talk about it in the meantime. There would be no different if they were reading a different magazine and talked about the articles.
What is Not Acceptable: Two or more people using their phones without talking to each other while they are out at a cafe.
What is Acceptable: A teenager using their phone while at someone's house they were forced to go. Before you start talking about impoliteness, I'll explain: It's okay if you visit your aunt's house to eat with them like they do every Sunday. The kid eats, listens to the adults for three minutes and then is fucking bored because the adults will continue talking about things the kid isn't interested in for four more hours. So why can't the kid open the phone and play Angry Birds? I bet no one would have cared if the kid was playing with a Rubik Cube but no , it's a phone so the kid must be stuck with it all day.
What is Not Acceptable: Having your relatives from America to visit for the fist time after 45 years and the kid is on the phone.
What is Acceptable: Hanging out with your friends and at some point take a snapchat/tweet/post something on Facebook
What is Not Acceptable: Being with your friends and constantly doing the above without paying your friends attention and/or talking about the above all the time.
I don't know if these are common knowledge or just me. But the next time you assume two teenagers in a cafe are "influenced by today's technology to isolate themselves" think about what I just said.