I've heard that links on Reddit go dofollow after something like 2 upvotes. However, none of my submissions have showed up on Backlink Watch. Links in comments have, though.
OP why would you care if these links are doFollow or not. They will not have Page Rank and therefore no difference of doFollow/noFollow. If you're under the impression that noFolow links somehow do not count, you are very much mistaken. A noFollow link from a high authority high traffic domain is very much beneficial to rankings. But Social Media links are best employed for what they are meant for... to share an interesting story, and get interested traffic to your site. Yes the link is nice, but the traffic is even better Your best sources will be : StubmbleUpon Digg Reddit Delicious Google+ Twitter Facebook
I use google plus for getting backlinks from social network, google pay much attention to google plus links.
storify dot com & chime dot in both are one of the favorite website of me because after posting you can easily get backlinks in search engine.
I'd say Google + is the best, since it is made by Google itself, but yes, Digg, Reddit and StumbleUpon are also good.