I doubt they are:
1) the volume of popular stories on digg/reddit/stumbleupon/hn etc cover a miniscule portion of the internet and aren't necessarily more important or better than the overwhelming majority of the internet.
2) Google can't trust votes/users, and in lots of cases can't even see who made the vote let alone decide whether user x on site y is legitimate. They also can't trust the sites to handle that themselves - they get gamed all day every day with fake accounts/paid submitters (even on HN!), manufactured content, summary spam etc.
3) every site implements/displays it differently which would mean scraping most of the time, and tapping into APIs only some of the time.
There are SEO benefits though like people linking to your site because of the popular submission, your site spending a day or two on a high-PR website's front page etc.
