Sgt_Kickaxe - 8:02 pm on May 16, 2012 (gmt 0)
I believe Google has compiled lists of sites which they believe are places your site SHOULD get links from if it's of sufficient quality. If your site isn't getting links from these places it may be a sign your content isn't worthy in their eyes. A Google post mentioning the addition of hundreds of forums as a source of social signals led me to believe this. The article was written in a "social discussions about your site" context but it makes sense that these places should be linking to you, it's where Google expects to find sites like yours mentioned. Are you being mentioned in the right places? The wrong ones are likely ignored imo. Focus on being mentioned where you should be instead of link quantity and you'll do just fine.
As Bing calls it - join the discussion about your site/content because it's happening with or without you.