I'll put this out there for discussion.
On April 24 2012, my traffic tanked severely. I'm not sure if the forum rules allow me to post a link to the graph, but the "search impressions" line from Google Webmaster Tools was at around 600,000 from March 1 (the earliest I had data when I did the screenshot) until April 1. On April 1, the impressions got really weird. They cycled up and down between 600,000 and 1,000,000 per day. That didn't correspond with any external activity in my site's niche (it is a sport). Then on April 24, bam! Impressions dropped to 250,000. Again, that didn't correspond with any external activity in my site's niche. It was about a 30-40% drop from the previous year. My site has been around since 1998, so I know my traffic, and this clearly was a penalty. Exactly on April 24. Penguin? Sure seems like it. I wasn't totally devastated because my site has a firm reputation and many repeat visitors, but it impacted traffic by 20-50%.
As my sport went into hibernation for the summer, the impressions dropped even lower, down to around 100,000 to 150,000 per day. Traffic was down by almost 50% from the prior year, though part of this was because the top league of the sport that my site focuses on was on the verge of cancelling their season.
Then, on October 13, bam! The impressions skyrocketed to 450,000. It can be argued that October is the time of the year when my traffic normally picks up, but the increase was too sharp for this to be that kind of increase. It was clearly a recovery.
One thing I had noticed was a hallmark of my penalty was that pages where my site should have been #1 (because I'm the only page on the topic) were coming in at #11. Not all pages, but long-tail pages without external links to them. My sense is that pages with a good number of external links were able to overcome the penalty. The difference between the affected pages and the non-affected pages may also have been the amount of information on the page - I was never really able to figure out the reason for the penalty. Still, I felt the penalty was sitewide. On October 13, that phenomenon disappeared and my longtail pages were again appearing at the #1 position instead of #11.
All summer I had posted repeatedly in the Google forums and the (mostly rude) posters there mostly focused on my content even though the penalty was on April 24. Consequently, most of the changes I made were to my content, site architecture, and internal navigation. I did ask a few sites that had linked to me blog-roll style to remove their links, but I never felt like my backlinks were a problem because I have so many of them and they are all natural.
To be clear, I have never bought, sold, or traded links, so this was a false-positive hit, in my opinion. My primary theory on why I recovered is that my site had a non-typical backlink profile which falsely tripped Penguin, and that on October 13 Google rolled out a fix that corrected their error. My secondary theory (which I hope is not true) is that there is a traffic threshold level for Google's penalties, and that from April to October, due to the seasonality of my sport, my traffic dropped below the threshold and lost its ability to overcome a penalty which might still be lurking out there.
I installed Google Analytics on July 1, and have been monitoring various factors since then. One thing that puzzles me is my breakdown between Organic/Direct/Referral. These three numbers have remained relatively constant from July 1 to the present. When I was in the penalty phase, they were about 62% Organic, 24% direct, and 14% referral. When the penalty lifted, they were 66% Organic, 24% direct, and 10% referral. The only variation I see in these numbers is from weekday to weekend, when my direct traffic increases due to people viewing my site on their mobile devices (mobile traffic goes from 35% during the week to almost 50% on weekends). I was expecting to see a much larger shift in those numbers considering that Google was now referring 30-40% more traffic post-penalty than pre-penalty.
TLDR: I wanted to put that out there - I was undoubtedly penalized on Penguin day, and my traffic undoubtedly returned on October 13.
1 year anniversary of penguin, no recovery [webmasterworld.com...]