We don't allow our kids to use social media. Some of their friends do, and some of them do not.
Doesn't seem to be a problem in any way.
Outside the classroom there are still plenty of in-person activities from informal gatherings, to sports teams, to clubs and civic events, and in those situations, the few people who are sitting on their phones are the ones more separated from the rest.
I think a lot of us who were early involved in building some of the web that led beyond Web 2.0 into algorithmically-driven media consumption see the massive dangers, and have probably sometimes even overreacted in terms of kids' access to it, but it's certainly better than the alternative.
Phones ARE banned from school and the kids DO socialize in person there.
But then they all get home and stay in touch. Some have phones, some have tablets, some just their school-issued laptops. But they're constantly talking, 1:1, in various overlapping group chats. They organize getting together in the neighborhood or meeting up elsewhere over the weekend. It's where a lot of the drama happens (which isn't great.)
If you prevent your kid from accessing this communication layer, it will 100% affect their social life.
Some degree of device pragmatism is inevitable of course. But I don't think my kids will need a smartwatch until high school, or a phone till very late HS. In other words, I think there is a balance of these options possible.
Jonathan Haidt gives a lot of hope here, as depressing as "Anxious Generation" usually is.
Social media does some harm. Being socially excluded does far _more_ harm. Which starts to happen in middle school.
So as a parent, you have three choices:
1. Deal with the emotional consequences of your kid being systematically excluded and having few/no friends.
2. Construct an alternative school/community with like-minded parents and kids.
3. Just give them the damn device, with screen time restrictions and the understanding that you'll be monitoring their activity. Follow through and be involved in their social life, with open and honest conversations about what happens online.
It turns out most good parents these days opt for #3.