Wonder why Netflix doesn't have this
As a trendy tech company I am a little surprised Netflix hasn't added 'social' functionality so you can have 'friends' on their system - recommend shows to each other, see your mates' reviews and comments, etc. Adding communal viewing would then be a natural evolution. Integrating their own chat, or Twitter, even VOIP (like multiplayer gaming but for TV).
I am not saying I WANT these features because I fear it would be a mess of "Jeff just watched Tiger King" but it does seem like something a lot of younger users would use!