This is an extension of the “travel face” idea where people fade out of your social network if you don’t “care” about their posts, and it adds everyone within 20m of you to your feed. So this is kind of a cross between facebook and that app that finds people to hook up with based on locality (I can’t remember it’s name).
Obviously when you’re somewhere busy it’s going to be chatty, but whatever, I guess people on trains and what not can have a “conversation” without having a conversation. Just a stream of posts from each other. People will eventually recognise each others posts and perhaps they’ll even become friends in real life.