Well here are some of the key points for you :
1. Bookmarks are Enduring.
While it’s true that sometimes someone will bookmark a page simply to come back when they have more time, more often than not the Delicious user views your page as a resource that they might return to again and again. A Digg is simply a one-time vote that may or may not result in big one-time traffic. A Digg is short-term event, while a bookmark endures.
2. Bookmarks Show Commitment.
Related to the idea above, a Delicious user has made a commitment to your resource by adding it to their collection of links, most likely because it provides a benefit to them. Other Delicious users notice that commitment, and therefore your resource gains in credibility thanks to that bit of social proof, which often leads to more bookmarks. I believe this is one of the key reasons why a big day on Delicious translates into subscribers and return readers, while the same exposure on Digg often doesn’t.
3. Each Bookmark is a Targeted Link.
Here’s the real power of social bookmarks. Each bookmark you receive is a separate link on a separate web page. Moreover, those links are classified by tags which define their overall relevance, much like normal link anchor text does. Not only does this help you in the search engines, it makes Delicious itself a search engine. Tagging will no doubt play a big role in the evolution of web search, and Digg’s model misses the boat here.
Extracts from Performancing Blogs..