Parent of a teen here. This is just flatly false.
If you have been a teenager or adult before, you will be familiar with the concept of the clique. For teens, there are athletes, nerds, theater kids, Lululemon kids, etc.
There are cliques of kids who do not use social media (because their parents won't let them, or they don't want to, or they prefer to do something else, or their parents do not use social media, or they cannot afford the devices). Teens who do not use social media sort into different cliques. That's it. They are not ostracized any more than theater kids or computer geeks are ostracized. (The latter inclusion was intentional, as it may cause some self-reflection among well-adjusted adults who at one time were members of school computer clubs.)