I know there's a fine line differentiating "social media" sites from everything else, but if a list like this is going to compare Flickr, Facebook and Blogger.c

I know there's a fine line differentiating "social media" sites from everything else, but if a list like this is going to compare Flickr, Facebook and Blogger.com as if they're in a single category, then I don't see why sites like YouTube or Digg aren't included as well.