How to...

use social bookmarking to promote your website

2013 note: this whole topic is now outdated

The thing with social bookmarking is that it works best to submit

Rules to social bookmarking

Delicio, yahoo-bookmarks and furl allow the addition of as many pages from any domain as you want - and have no way to punish people who put in too much of the same stuff or even spam. Still, even in these cases the promotional effect will be better if you act human and don't submit spam. People won't add your account as a 'friend' unless they see it as useful. They will not see it as useful if you are too promotional. 

With stumbleupon and Digg the effect is better if you have friends. Getting friends interested in your niche(s) will happen only if you don't submit spam and do submit only limited stuff from your own website. Some say a good guideline is 10% of your own stuff versus other people's. 

Digg has a very specific implicit rules on what to submit and only power-users are likely to get any benefit out of it. This is to do with the internal culture, and the ease with which people can vote articles out of the index. This is called 'burrying'. Conversely to get articles on the main page of Digg, it should not only be well done, you also need a lot of friends on Digg. This makes it useless to just submit your own stuff and never look back. That will not get your website noticed. Because getting material noticed on Digg is so difficult, power users are actually getting payed to submit other people's stuff.

Stumbleupon, like many other social networking sites,  has internal (programmed) rules on not submitting too much from the same website. The precise dynamics are different, but the effect is the same: be social first and don't submit too high a percentage of stuff from your own site.

Niches

A niche is a subject.  Delicio , Yahoo bookmarks and Furl can be used, for the reasons mentioned above, for any niche.

Stumbleupon works for any subject that interests people. Arcane, unpopular - as long as you aren't the only one interested in something, it can be found through stumbleupon. For promoting websites this means that any good website can be promoted through stumbleupon. Most other social bookmarking services are more limited. 

Digg has a technical culture yet SEO (Search Engine Optimization) cannot be promoted through Digg. Famously Apple news will be popular while right-wing politics will be buried. 

Conclusion

Social bookmarking is a human endeavor. Those willing (and able) to invest in getting to know the culture of a specific social bookmarking service will reap better results from promoting there, then those who just submit their stuff through say an automated social bookmarking submitter. This is more true for Stumbleuopon, Digg and Netscape then it is for more bookmark (and less social) services like Delicio.us, Yahoo bookmarks and Furl.