Bit of a shameless plug this but i wouldn't post if i felt it wasn't valid.

Social bookmarking sites like Digg can still be a huge part in a sites promotion and link building.
The key in my opinion is to go niche, then you know your site is 100% relevant to the readers and voters. It gives your site a standing point in the niches community ad makes it well known to popular bloggers etc...
Look how great Sphinn: News, Discussion Forums & Networking For Search & Internet Marketing Professionals is, it brings together the internet ,marketing community and is a daily read for me.

Focus on getting links not from the social site but others reading it, you need to be trying to get the voters to blog about your article or site, look at who the powerful users are and interact with them, try and get them to publish your post or vote.

My own growing list here Niche Social Bookmarking Sites and il be adding some descriptions of the site, PR, dofollow etc..
Always looking to add to it so let me know if theres any others you use