The deference between social bookmarking sites and what we called websites similar to MySpace is that in the social bookmarking sites you cannot build a profile page, you can only bookmark your favorite websites. Your links will be considered as a high quality back links.
If you have a new website, or webpage and want to index it fast in the search engines it's better to add it to high ranking bookmarking site than submitting it to the search engines.
Google for instance is crawling the Digg site in high frequency, so he can index new websites from Digg faster than any other way. Read the full article