Interesting question Justin.
I think when we work on the internet, we use social bookmarking sites, like Digg, Stumbleupon, del.icio.us, and all the rest, as a matter or course, but sometimes we forget that most people browsing the web haven't heard of any of them!
Personally, I use most of them, not only to save my bookmarks and 'stumble' sites I like, but also to put my own blog posts 'out there', so that people can find them. Of course, the only people who find them will be those who know about social bookmarking...
Nevertheless, there are an awful lot of people using sites like Digg and Stumbleupon, as I found out the other day. I posted a link to Stumbleupon from a post I had made on my
Depression Help blog. It was a good post (written from the heart actually) and I thought it was well worth reading. Then I got on with my other work...
I went back to that blog an hour later and found I had received over 3,000 hits from Stumbleupon in that last hour. And I sat and watched as many more came in. I even began to get a little worried about bandwidth...
Considering that experience, I for one will be 'stumbling' and 'digging' more often than I used to. And from the perspective of those who want to find new information without having to scan through Google search results, social bookmarking can be useful too, especially sites like Stumbleupon, which allow you to save images as well as site links or browse other peoples' collections.
Can take over your life if you are not careful though...