In an article for SEOmoz.org, Jane Copland describes 30 popular social media sites and their potential for driving traffic to your website. She lists the sites in rough order of "usefulness." For each site she describes the Website's "pitch" about what it should be, and then gives her evaluation of how people actually use the Site, and how one might use it as a tool to get more traffic.

Below are the top 5, with excerpts of the author's description.  It is interesting to note that StumbleUpon is listed at number 11. A recent informal experiment by the Social Media Optimization Blog (http://social-media-optimization.com/2007/06/what-social-media-sites-send-traffic-review/)  and web traffic analysis by Darren Rouse of Problogger http://www.problogger.net/archives/2007/07/06/why-stumbleupon-sends-more-traffic-than-digg/) both concluded that StumbleUpon sends more traffic over time than other social media sites, including the number one choice here, Digg.

1. Digg

• You can submit your own content to Digg; however, whether your content is dugg en masse has a lot to do with the status of the person who submits the article. Think of it as Washington DC. You want to get an influential lobbyist to work on your behalf, because even if your initiative is awesome, no one will listen if you have no reputation.

...

• You cannot have everyone in your office digg what you've submitted. The site's editors will notice that a lot of diggs are coming from the same IP address, and their most likely course of action will be to ban you and your workmates' accounts. Trust me. Of course, having a bunch of different usernames will achieve the same result.
• If you're dugg and your servers can stand the load, the traffic you gain from such an occurance is priceless.
  

2. del.icio.us

• The "network" feature of del.icio.us may well be its most useful tool. Users can identify other people whose content they respect and appreciate, thus becoming privy to the bookmarking actions of those users. It's also possible to view the people others have added, and those who have added them.

. . .

• Hopefully, those people whom you add to your network will browse your content also, and things that you bookmark will become more visible. You can start to develop a community of people who share your interests.
• This community is one of the main things you can get out of del.icio.us that differentiates it from similar sites.

3. Reddit

Another social bookmarking site, Reddit lets you save and comment on news stories that you or others submit to the site.

• Reddit will send you more traffic than you may think.
• It seems that Reddit's users are, on the whole, older and smarter than Digg's. This is not to say that Digg is full of idiots; however, the average comment at Digg seems to be less constructive than those on Reddit. Popularity probably plays a part here, as Digg is dealing with a bigger audience than is Reddit.

. . .

• There appears to be no bias amongst Reddit users towards technology, current events or anything else. The homepage can contain stories about any subject, as well as cool pictures, science news or anything else.
• This diversity means that all topics have a chance of being voted up, but your topic is just one amongst many.
  

4. Technorati

Without a coherent plan of what one expects to achieve on Technorati, this site seems a little daunting. The most useful tactic is obvious: you must add the blogs you like as favourites in order to take advantage of the site's huge database. Additionally, searching for keywords can be fun, but you will soon be cowering from the scores of MySpace and LiveJournal blogs who have mentioned said keyword in their rants.

• Keep tabs on your online visibility using Technorati.
• Technorati will show you who's linking to your blog, which is a great service.

. . .

• Technorati should be used in conjunction with other marketing tools. Although your potential for direct marketing on the site is limited, it will show you how much progress you're making, where you're being mentioned and how your competitors are doing.
  

5. Squidoo

While their attempt at wit and ingenuity in their pitch only confuses Squidoo's purpose, the website itself is a good resource. Letting people create "lenses" about any topic, Squidoo is different to LinkedIn in that people are not limited to promoting themselves. As well as individuals, businesses, services and hobbies are all popular Squidoo lens topics. To clarify, a lens is a page created by a user, and users can create as many lenses as they wish on as many different subjects.

• Squidoo actually promotes the addition of external links to lenses. In fact, in their FAQs, they specifically show off their link lovability, stating that Squidoo lenses have "huge credibility" with search engines, and briefly explaining how this can help the sites you link to rise up Google searches. No nofollowing here, people!

. . .

• Lenses that make it to the top of the rankings (#1 - 100) are invaribly text, link and picture rich, although because the rank is assigned by a computer, the lenses with the top rankings are not necessarily those with the most useful content. For example, the premier lens when this article was being written was "Funky, Chic and Cool Laptop Bags." Cute, yes. Useful? Not really.

. . .

• Include enough links to your actual website that people will see many opportunities to click through. After all, anyone who is interested enough in what you have to offer that they bother reading your lens is probably also interested enough to see your site.
• The main thing you have to gain out of using Squidoo is increased online visibility from a nicely presented source that is not a wiki.

This is not a wiki and thus, the content is entirely up to you. It is a good idea to have a Squidoo lens to you company's name. Done well, your lens will appear in search results.