What is it?
Social bookmarking is the practice of saving bookmarks to a public Web site and tagging them with keywords. To start a collection of social bookmarks, you need to register with a social bookmarking site, which lets you store bookmarks, add tags of your choice, and designate individual bookmarks as public or private. Some sites will, from time to time, verify whether the bookmarks still work,notifying users of a broken URL. Visitors to social bookmarking sites can search for resources by keyword, person, or popularity and see the public bookmarks, tags, and classification schemes that registered users have created and saved. So, it is particularly useful when collecting a set of resources.

Because social bookmarking services indicate who created each bookmark and provide access to that person’s other bookmarked resources, users can easily make social connections with other individuals interested in just about any topic. Users can also see how many people have used a tag and search for all resources that have been assigned to that tag. In this way, the community of users over time will develop a unique structure of keywords to define resources; something that has come to be known as a “folksonomy”.

So here is the list of social bookmarking sites arranged by Pagerank (PR).

PR9:
http://www.netscape.com/
http://slashdot.org

PR8:
http://digg.com/
http://del.icio.us/
http://www.furl.net/
http://www.stumbleupon.com/

PR7:
http://reddit.com/
http://ma.gnolia.com/
http://de.lirio.us/
http://myweb2.search.yahoo.com/
http://www.simpy.com/
http://www.newsvine.com/
http://www.spurl.net
http://www.netvouz.com
http://kinja.com

PR6:
http://bluedot.us/
http://blogmarks.net/
http://www.blinklist.com/
http://www.netvouz.com/
http://www.shadows.com/
http://www.dzone.com/
http://www.clipmarks.com/
http://www.shoutwire.com/
http://www.backflip.com/
http://www.bibsonomy.org/
http://www.diigo.com/
http://www.listible.com/
http://www.blinkbits.com
http://bluedot.us
http://www.plime.com
http://www.blogmarks.net
http://www.marktd.com

PR5:

http://www.connectedy.com
http://feedmelinks.com
http://www.lookmarks.com
http://www.bookmarktracker.com
http://buddymarks.com
http://www.butterflyproject.nl
http://www.markaboo.com
http://www.ikeepbookmarks.com
http://www.mylinkvault.com
http://www2.myhq.com/
http://www.myvmarks.com
http://www.oyax.com
http://quadriot.com
http://my.xilinus.com
http://www.tallstreet.com
http://www.tagtooga.com
http://otavo.com
http://www.blogmemes.net
http://www.30daytags.com
http://www.givealink.org
http://www.web-feeds.com
http://www.zoogim.com
http://www.putvote.com
http://www.folkd.com
http://www.gabbr.com
http://web2list.com
http://www.wirefan.com
http://www.listerlister.com

PR4:

http://cloudytags.com PR4
http://www.a1-webmarks.com PR4
http://i89.us PR4
https://www.chipmark.com PR4
http://complore.com/ PR4
http://linkatopia.com PR4
http://public.sitejot.com PR4
http://www.syncone.net PR4
http://www.urlex.info PR4
http://www.chaamp.com PR4
http://www.meme-stream.com PR4
http://www.getboo.com PR4
http://go.lilisto.com PR4
http://imp.etuo.us PR4
http://www.theadreview.com PR4
http://adveracio.us PR4
http://www.startaid.com PR4
http://www.oks.ph/ PR4
http://www.scooop.net PR4
http://www.barksbookmarks.com PR4
http://www.business-planet.net PR4
http://www.futurosity.com PR4
http://www.votelists.com PR4

PR3:

http://www.inchicks.com PR3

via digitalpoint