Have no idea if there's the nofollow attribute but it's true you'll get good traffic if someone stumble your site.
Neither will significantly increase backlinks - but both can benefit your site traffic. Stumble especially I've found to be a great source of visitors for entertainment related sites - just remember that most stumble users seem to use the service when bored, so provide them with something interesting or fun and they'll use the thumbs up feature and traffic can rocket! I have a certain entertainment site that gets 60% of it's traffic from StumbleUpon, and I originally only added a few pages to Stumble.
Sites like del, stumble are not supposed to give backlinks and hence use no follow attribute. If your site has interesting pages, you may get traffic from stumble.
if you will get more traffic from stumble or del.icio.us then from Google why not to use both of them
del.icio.us will give you a low volume of targeted traffic, whereas stumbleupon will give you a short burst of untargeted traffic. Unless your landing page has immediate appeal, stumblers will leave (and from what I've heard, they rarely click on ads).
Seems that you did not use it Calling it untargetted is not correct. If someone is not interested in "Cars" then it is not directed to car related sites. So alhough it is not as much targetted as Adwords like methods, it is still targeted! And, yes currently the user base consists of tech guys mainly who don't like ads much
well the point in both sites is not to gain backlinks but to gain traffic only ever used delicious tho but you need alot of people to tag your site