Also there is a chance of you being wrong because, even though there is no nofollow tag at the link place, but if the site is using a robot.txt file for making the links nofollow then also we dont get our links indexed, right?
You should have checked the robots.txt of digg before you said that. But your point is right. They can use a nofollow in their robots.txt to stop google from following your links. But in that case they will be blocking their own links as well.
Go through this nice link, there are so many list of social book marking websites link. Dofollow Social bookmarking website
if you want to post links to bookmarking socials sites, try to use Bookmarking Demon software. (bookmarkingdemon.com) But don't forget, that spam is not good... sanjai2001, thanks for the compliment