- It's fun, and the toolbar is actually useful.
- It's easy to integrate into your blogging/surfing habits.
- It connects your blog to a larger community.
- The 'Stumble Effect' is arguably better, and far less painful than the 'Digg Effect' (traffic over time as opposed to all at once)
- The friend/contact mechanism makes much more sense than, say, MyBlogLog's.
- You get ME as a buddy. (This clearly should have been #1)
The Basics: There's really not much to using StumbleUpon (I like this). There's a toolbar you'll use to find, vote on and review new pages, and there's your profile page which includes a mini-blog that catalogs your Stumble activity.
The Toolbar: The main functions of the toolbar are the "Stumble" button, which takes you to various websites within your areas of interest, the thumb up/thumb down buttons (i.e, 'this is cool'/'this is ass') and the 'reviews' button, which lets you see the reviews of a page and/or add your own.
The Profile Page/Mini-Blog: This page describes who you basically are and shows (via the mini-blog) the pages you think are worth visiting.
If you aren't regularly writing reviews for the pages you like/dislike, you are going to look fairly boring, as your mini-blog is just going to be a list of post/article titles; and if all your recommended pages are from your own site, you're just going to look lame. If you are writing reviews of the various cool sites and articles you find (including your own site's best stuff), your personality will be much more apparent, leading people to follow your links, vote positively for you, and add you as a friend.
About Friends: Where friends and contacts currently mean very little on a service like MyBlogLog, they're highly significant in StumbleUpon. Who you choose as friends will determine much of what you're shown when you click the 'Stumble' button, as you'll be seeing the pages they've voted positively for. Likewise, if they befriend you in turn, they'll be seeing your submissions.
Since you have a limit of 200 friends at any given time, spamming the friend button is counter-productive. If they don't have similar interests, you're going to be seeing a lot of pages you aren't interested in; and they're highly unlikely to make you a friend in return.
And More: This is a good guide to promoting your site with StumbleUpon.
Social networking is all but required for promoting your site these days. There are right and wrong ways to do this. Don't look at StumbleUpon as merely a tool for self-promotion; instead, put in the effort to actually join the community. Winners do it.