Netflix should build their own social network, for the heck of it. I like having different social networks for the various public spheres I participate in, instead of trying to manage 'one social network' for all of the public spheres I interact in.
/edit: Moreso, if everyone has a free Netflix account to rate films, then they're already used to your site when you push for sign-ups.
Not just for the heck of it, it makes sense, they're in a perfect position to build last.fm for movies/tv. They could even run an open video scrobbler and lure in pirates little by little.
I honestly don't know if that even still exists, and netflix.com seems to be giving me errors tonight so I can't check. Now that TVs and game consoles can get there and I no longer have a disc plan at all, I rarely visit the site in a browser. The various set-top-box clients I've used don't expose that functionality at all, so it may be gone or relegated solely to the browser, which at this point feels like the backwater of the Netflix experience.
Edit: Would be great to have the social media element be integrated into the TV and console apps. I to love these apps, and would love to find and follow other similar users and friends on the Netflix network.