diberry - 4:49 pm on Sep 7, 2013 (gmt 0) [edited by: Robert_Charlton at 6:07 pm (utc) on Sep 7, 2013]
I can back up the observation about social media links with some anecdota.
I have one site hit by Penguin 1. I didn't do a lot with it after that because I honestly didn't know what to do - I hadn't built links, hardly had a link profile at all, didn't get an unnatural links notice. I made some structural improvements and left the content mostly alone, but began working harder to build a good, valuable-to-visitors Pinterest page.
One page in particular took off with Pinterest - it was just right up a lot of users' alleys. My rankings on that post are very good for quite a lot of relevant key phrases. This improvement started before Penguin 2, so it can't be that Penguin 2 released me from penalty.
Another interesting anecdote: I have another page that has proven extremely useful to visitors (it details a method for dealing with a common scam). This page hasn't gained any new links or social media response. Desperate searchers just keep drilling down the SERPs, looking for solutions, until they find my page and the information on it actually works. Then they often leave comments telling me it worked. So either Google is detecting that searchers go through lots of pages before mine and then stop after getting to mine; or they are somehow detecting phrases like "This worked" and "thank you" in my comments (which are not hidden from the bots). Either way, this suggests Google is at the very least finding roundabout ways of generating user metrics and using them to determine rankings.
I should mention that I never built links and in fact my site hardly had a link profile before Penguin, so no one's quite sure why it was hit by Penguin. Therefore, my results be different from someone who built links and then used disavow.
[edit reason] spliced from split-off discussion [/edit]
[edited by: Robert_Charlton at 6:07 pm (utc) on Sep 7, 2013]