Social bookmarking is getting more and more popular everyday on the Internet.  When people social bookmark your site, you can get that many more visitors directed to your website.  There are a bunch of plugins for WordPress that help you bookmark your site.  For a list of such plugins, you can check out the WordPress site yourself:

http://wordpress.org/plugins/tags/social-bookmarking

As for external bookmarking sites, don’t forget to social bookmark your site on Facebook, Twitter, Google Plus, Pinterest, Tumblr, Reddit, and Digg.  The best part about social bookmarking is that if people like your website, things could go viral and then you get even more visitors to your website.  Just look at how things go viral on Twitter and you can see how some websites get a huge boost in traffic in a very short time period.  Also, getting your website bookmarked means you reach out to a larger audience than you would have otherwise.

I would recommend Twitter and Facebook first as those are the two biggest social media platforms now.  If you can get people to tweet and retweet your website, you will reach that much larger of an audience.  In Facebook, people can share your link so if people like your website, they will share it to their friends who may in turn share it to other friends.  Just have a website that people like and they will naturally share your website in time.

Best of luck on your social bookmarking and may you get a tonne of visitors to your site!