There is no doubt that backlink from Digg helps in quicker Google indexing but it is also important that you post something useful on digg that other people will find interesting and vote for you. Social media can help getting quicker results on Google, although these results only give short term boost to your SEO efforts.
I got some trouble on digg, eg. my digg page's rank (digg.com) replace the real page's rank (the site that I submit bookmarking) on google. I can get only 1000 friends on digg so I prefer stumbleupon and twitter.
I use digg every time I start a new site. I have compared the differences of using them and not using them and it deffinitely helps. As mentioned here it does help to get quick initial traffic and helps to get listed quicker.
Digg is do follow, just search up the topic that is releavant to your niche, and comment on those pages, a lot of page rank 1 to 5 pages there!