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Keep in mind when viewing the sites that are under "Bookmarking" there are multiple reasons for them being here. You have to think outside the box and realize that all of these sites can be used for much more than what they say on the site. Social learning and networking is all about combining your resources and finding new ways to communicate using these sites. Also, listed below may not be all of the possible sites that could fall under "Bookmarking", but here a few to get you able to see how they can be used for such a purpose.
Example: Why can't Wordpress be a way to bookmark your sites? It can potentially be anything you want it to be, no? You could have a section on your blog where you place your favorite bookmarks under so your friends, families, employees, whoever may read yoru blog, sees your bookmarks and can view them also. You make it what you want.
Refer to Articles section for more ideas and viewpoints on how other people have used these sites.
Delicious
Digg
Diigo
Stumbleupon
Wordpress
Activities
I personally use Delicious. Its a great way to keep all of your bookmarks with you wherever you go. It alerts you a lot when adding new bookmarks, but I wouldnt say too much to where it gets annoying. Even when deleting your cookies and history, it still keeps all of your bookmarks if you would like it to. Its a very organized and simplistic way of bookmarking your favorite sites. Try it out and transfer all of your current favorites to Delicious, and if you dont like it, you still keep the favorites you added to Delicious.
Digg is a fun site to go to and express your opinion on the newest and most popular articles. You can bookmark the articles to to your aggregators, blogs, and bookmarking tools. Digg is very user friendly and easy to use. It's a good way to gather articles based off specific topics, and gather future articles on those topics as soon as they are published.
Articles
Check out these articles to give you an idea on how these resources can be used for a lot more than they may seem. You might find out ways to use some of these social networking sites in ways that can really help you.