Do comments on social sites like MIXX and DIGG the same as commenting on blogs when it comes to link building (link juice).
The comments themselves? The comments don't matter in your SEO work. It's where you leave a link. For me, I wouldn't leave a link from Digg since it is a NF site.
if you make a comment, then your social media profile will be used as hyperlink to show who made the comment. this will not help you in terms of getting backlinks. you need to actually bookmark your site to get the backlinks and SEO benefit