Just re-read this article and decided I wanted to share:
Great little overview of why & how these sites fit into a social media marketing strategy.
For those who don’t know, del.icio.us is a social bookmarking site where you save your bookmarks to the Web, tag them, and share them with others. StumbleUpon is a cool way to find new Web sites on a particular topic by subscribing to the topic (tag). StumbleUpon then helps you “channel surf” the Web on particular topics. In the case of both sites – it pays to have a network of ‘friends’ that pay attention to your content, and you can use both to reach the top influencers & early adopters in a particular niche. Both can send a lot of targeted traffic and help you build links.