A slow but very interesting WebmasterWorld thread asks, "Is Googe Devaluing "Social Content" Links?" Tedster, WebmasterWorld Admin, said;
It's not a big secret. Webmasters looking for a ranking boost have been using "web 2.0" tactics for quick link building. Even folks with long-established sites began to dabble in this trendy area. It sounds pretty innocent, doesn't it? The question is being asked even by major corporate web teams. While this trend mushroomed, I've been wondering how long it might take for the Google algo to defend against this kind of link, which really is not the kind of "earned vote" that they most want to reward. So, has the time arrived?
Now, Google tells us to "focus on the users and not on search engines when developing your optimization strategy." But it is my understanding, that the point behind these social sites are that they are the human vote. To get to the top page of Digg.com, please need to like your content, and if people don't then you won't get on the top page. Why do SEOs want to get to the front page of Digg? Well, not only does is send a lot of quick traffic, it also encourages Digg users and others to blog about your content. That means more links. More eye balls, more links.
So you have two obstacles here. Assuming that a link on Digg.com in the eyes of the search engines may be valued at a normal level. The SEOs first need to get their content on Digg.com, ranking well. The second thing is they need to have people write and link to their content on their own sites.
Forum discussion at WebmasterWorld.