Yeah the dofollow SBM sites are great for PR and ranking whereas nofollow only pass a very limited amount of link love.
nofollow tag will not pass any juice to your website nor will it help your site in getting a high PR . A dofollow tag will pass on good juice and PR and search engines will count that a link
Most of the SBM sites have the no follow attribute enabled and hence the chances of getting a higher PR from these links is nil. Further the traffic from these SBM sites are not targeted and hence monetization of the traffic is difficult.
From my experience, it doesn't work very well, but it can't hurt. Do some research to see which social networks worked for others so you don't waste your time!
I think most of the Social Bookmarking sites use "nofollow" tag.. Use them more for traffic and publicity rather than building backlinks..
Do a google search for DoFollow Social Networks. There are quite a few. I've used social networks that follow with very nice results.
It's a little bit to find the "dofollow" SBM sites, but it will be useful once you've found several of them...
they are okay link providing that they are from do follow sites, however i find them more useful for traffic rather than as good backlinks