Reddit is definitely the best of breed combination of Social Networking and Social Bookmarking. I personally find I get at least 5-10 times the traffic from articles submitted to reddit than I used to get with Digg. If you don't have an account, you should definitely check it out...
Reddit is just a collection of links that people vote on what's the most popular. Same thing as Digg but Reddit is less serious than Digg. Too bad that's becoming a big problem with Reddit.