But humans don't, in any meaningful sense, interact with 7 billion people when they use social media any more than they interact with the entire population of their city whenever they go out. And most people living in any reasonably sized city - to use that as a real world analogue for social media - aren't only interacting with small, high trust social networks of the same culture and beliefs, and they manage just fine.
Your hypothesis (which seems more and more common) seems to me to be a "just so" story, but it doesn't correlate with what I've observed of real human behavior.
1) People choose to live in the same city so they have that in common
2) 7B don't all interact together, thats not the point. The point is that its random who you talk to as most social media is anonymous.
Combine this and you have low trust + low chance of having similar viewpoints/culture/beliefs.
Manage just fine does not mean everyone is happy, and in these major areas like NYC people always seem to congregate with the groups they share the most in commong with.
Your hypothesis (which seems more and more common) seems to me to be a "just so" story, but it doesn't correlate with what I've observed of real human behavior.