Letterboxd is a fairly new social networking site I've been writing on recently. Specifically for movie lovers, it allows you to keep a running diary of films you're watching, along with the ability to rate them, "like" them, and write short or long reviews. Like Twitter, it allows you to follow other people (and they you) with the ability to comment on their reviews like a blog and even create lists (both private and public) for any purpose you'd like.