StumbleUpon is worth taking a look at. With a community of only 11+ million people, the number of current useres is tiny compared to Twitter and Facebook, but I think it is worth your time.
The premise is similar to the Facebook "like" buttons or "diggs" if you are familiar with Digg.com. What sets StumbleUpon apart from the others is the ability to "stumble."
When you set up your account, which only takes a minute, you pick a few topics you are interested in. When you "stumble," StumbleUpon brings you to pages that they calculate you will like. You can give that site the thumbs up, the thumbs down, or stumble onto the next site. The calculation on which site to send you to is done by the computer and takes into account your interests, the past websites you have favorited, and the websites that other people with similar interests have favorited.
Kind of like with Pandora, where you start off with a single song that eventually morphs into a custom tailored radio station of your favorite music based on your interactions with the website, StumbleUpon seems like it should be able to deliver interesting reading.
For you bloggers out there and other contributors of Web 2.0 writing, photos, and/or video, StumbleUpon lets you add your stuff to their list of items that can be stumbled upon. Will people like it? That probably depends on how good it is. But it should put your stuff in front of new targeted readers.
I've used Digg before, but correct me if I'm wrong, I don't think Digg had this stumble feature, which I think is the best thing about StumbleUpon. Other than that feature, the website seems very similar to Digg, albeit a little more user friendly.
So will it drive traffic? We'll have to wait and see. I added my post about Miley Cyrus kissing Justin Bieber. If that doesn't go viral, maybe I'll try posting the Killer Saipan Tsunami video.