I dont really think that social media traffic is useless. It does create linkbait if your content is really good. I had once come up on sphinn and I got like 400 visitors. Average visitor time was like 3 minutes. Sites like digg and stumbleupon wont give you the results, if you compare it with your efforts on these sites. But if your server has the capacity to handle that much traffic there is no problem in using them. Depending on the content stumble upon can get you around 250 to 1500 visitors and digg can get you from 25,000 to 1,50,000 visitos in a day. It depends.