It is easy to submit sites on all these sites. In Twitter you can easily share the website by tweet. In Digg as you have account there is option Submit Link to submit website. Same is in Stumbleupon too. And most important all these websites are popular websites and are good source of traffic.
You can try on ping[dot]fm in generating backlinks and able to share/submit your content.Ping[dot]fm is one of the best free tools to use for saturating your articles, videos or press releases on the internet. I'm actually using ping[dot]fm to post updates about what I'm doing, new content that I published, and sharing to all of social network sites that I joined like twitter, facebook, digg, etc.
its the easiest process i think. just after visiting and creating the accounts you can dig, or submit your links in these sites. nothing complex.