Here is a quality post from Graywolf’s SEO Blog about how to help your visitors bookmark your content on social media sites.

There are, of course, other ways to make it easy for your visitors to bookmark your site. Here are some of the better ones:

• If you have a Wordpress blog, use (as we do) Semiologic’s Subscribe me plugin.
• Consider using Ekstreme.com’s Socializer.
• Addthis.com has just come out with a couple of buttons to simplify the whole process. Check it out here.
• Finally, don’t forget the Social Bookmarking Creator at Online Marketing Blog.

Whichever tool you use, it’s a good idea to make it easy for people to add your site to social media sites. Unless, of course, you aren’t really intersted in targeted, free traffic.