We’ve all heard of Facebook, MySpace, Twitter and YouTube — these are different social media vehicles marketers are using to extend the reach of their brand or product. While the world is atwitter about Twitter, we’d like to bring your attention back to the social bookmarking site we mentioned in this newsletter last June: StumbleUpon.com has now surpassed 7 million users, 50% larger than Twitter! What is StumbleUpon and social bookmarking? Instead of going to your favorite search engine, you simply register at StumbleUpon, tell it what you are interested in (travel, music, politics, cooking, etc.), hit your "stumble" button and sites approved by users with the same interests will be served. Then it’s your turn to give the site the thumbs up or thumbs down, which will then help the next user. Why might you care about StumbleUpon? Because it is a proven traffic-driver for your site or blog and, for a nominal fee, you can have your website inserted into the “stumble stream” and served to users with relevant interests. If your site is engaging enough, prepare to see StumbleUpon as one of your top traffic referrals.