I tried creating account on all of them, worked hard but at last i was only able to get traffic and some response from Facebook. I think its doing good, being harsh at times which helps it to make quality...
Use Twitter, I get traffic from my followers every day. Start looking at Twitter users that are interested in the same topic as you and add them to your account. That's how you get followers. Sign up for Twitterfeed too - it will aggregate your RSS feed into a Twitter status update so when you post something new, your Twitter will automatically get updated as well. Great time saver.
I also get some traffic from twitter...but well it has very low conversionrate as compared to email promotion campaigns
It seems all my traffic is coming from natural results from search engines and direct referral frmo other sites and some traffic from twitter, facebook , stumble and digg. Each and every case is different in how much traffic they are getting from stumbleupon etc.
It depends on how you utilise them! Facebook ads can be very profitable more so than any of the others because you can run an actual ad. Tweets are limited to 140 characters and if you are too commercial on them your response can be limited. Stumbleupon and Digg bookmark other content and their effectiveness depends on the popularity of your content. For my money I would go with Facebook!
how i wish you raise a poll so the results is tally properly. I haven't tried it yet and waiting for the result which is better.. Hmmm.. maybe its better if i tried it all anyway its free.