Even though I am a moderator here what I am about to say is
my personal opinion on this subject and this is an area where I would prefer to add some distinctions to Ed's message.
Social
bookmarking is the process of storing your browser bookmarks on a website such as delicious. These are public so they create backlinks to your content which is great and because they are your bookmarks nobody can complain about what you bookmark.
However, Digg, Propeller, StumbleUpon etc
are not bookmarking sites - the first two in particular are
social news sites and SU is a discovery tool. What they all have in common that is distinctly different from bookmarking is that when you share content, you are implcitly asking other people to vote on it.
Digg don't like sites about vintage guitars, or underwater kickboxing, or toenail clipping. Nor do StumbleUpon. Well only if the content is really good and adds solid value - the stuff out of EZine articles does not usually fall into that category.
Now StumbleUpon is a different beast entirely and its algorithm takes into account the
ratio of sites that you stumble and I absolutely guarantee you that if all you do is stumble one single site you will be
banned. Please, heed the warning - YOU WILL BE BANNED from StumbleUpon if all you do is stumble your new niche sites.
I have written more about this in the following blog post and that links to others that I wrote in the past. I hope this helps clear things up and I hope Ed doesn't take offence
[edited out as self promotion as wonderful as Caroline is - not within 30DC rules]
The advice to actually bookmark all your content is sound - its an excellent strategy but please understand the difference between a bookmarking site and the other types of web 2.0 content sharing sites.
Now having said that, StumbleUpon is an excellent source of traffic and if you write good content on your niche sites then you can absolutely submit some of that to SU but you must also stumble other stuff. I have written a course all about that, I won't plug it here - I'm sure you'll find it if you are interested.