myspace, orkut etc are social networking websites and digg, propeller, reddit etc are social bookmarking website!!
In my opinion, social networking mainly connect you with other peoples, social bookmarking website mainly connect you with some websites.
Social Networking: Sites to network with PEOPLE around the world. Myspace, Facebook, Orkut, Hi5 Social Bookmarking: A public forum where you bookmark sites you find interesting for others to see. If they find the site interesting too, they endorse it. The more endorsements a site has, the more it is displayed for others to see. Digg, Reddit, Stumbleupon etc. These two things are not similar in the least.
i believe ganpat is more than correct. thanx ganpat i am fully agree with u but was not able to differentiate between these two. more suggestion are welcome