It's not smartphones or "straight to the bedroom". Online communities for teenagers have been around in some form since the 80s.
Now everyone is expected to be on social media and it is the main community for a teenager, not an escape for nerds. If you're not successful there, you're also an outcast in reality.
This exposure wasn’t mainstream until the early-mid 10s when every high schooler got a phone and social media. Before then, it was computer and game system based and that changes.
I don’t know real screen time data, but the use of phones probably added an extra 4-10 hours of screentime per day. And the type of activity is very different with notifications and pickups being top of mind.
Now everyone is expected to be on social media and it is the main community for a teenager, not an escape for nerds. If you're not successful there, you're also an outcast in reality.