List of social networking / bookmarking sites

Here we are talking about sites like Digg (236 million annual visitors) and Reddit. They tend to combine social networking with story aggregation. And as the web has filled up with so much rubbish they have taken on an important role. They give the users the information they want far more accurately than search engines do. If you have a story that is good enough, like cream, to rise to the top then getting it onto these sites is incredibly important. They reach vast audiences.

It is easy to find the most important social networking sites because their logos lurk at the bottom of every article on world class sites like the BBC (this alone tells you just how important they are):