Sitting in a dark theater filled with random strangers, not moving, not talking, is not my idea of a social experience. It's antisocial. Watching a movie at home with a few friends can be a social experience. You watch it together, enjoy it together. But in a theater, everyone is alone. You're surrounded by people, but you can't interact with them, so it's just you and the screen.
Talking about it afterward is social, but you can do that whether you watched it at home or in a theater. And the movie itself is a lot more social at home where it's just you and your friends, and you can all see each other, and you can talk exactly as much or as little during the movie as you all decide you want to.