I always find it interesting when people come into the Google Webmaster Help and complain about issues with Google and it turns out they were obviously doing something against Google's webmaster guidelines.
The webmaster said:
I checked my pagerank recently (I hadn't done it for a couple of months) and was surprised to see it dropped from 6 to 3.
I haven't done anything differently since I created my blog some 10 years ago, i do have a couple of ads but I had them for a long time and stayed at PR6 for years.
I do have also a lot of pictures but that's part of the interest of my stories. My readership (which is high) doesn't seem to fall with the new lowered ranking but still, it's surprising.
Google's response from John Mueller was that he was selling links and passing PageRank without adding the nofollow.
I'd recommend checking the Webmaster Tools manual action section to find out more. In this case, it's a matter of links on your website to other websites, for example the "Wholesale Home Products" link in the sidebar. It's fine to have ads on a website, but to avoid those ads from skewing search, we recommend using the "rel=nofollow" markup to prevent them from passing PageRank. If you choose to change that, then you can let us know about this in Webmaster Tools (in the manual action section).
You can find out more about this at https://support.google.com/webmasters/answer/96569?hl=en
I am not sure why this surprises me, but it does.
Forum discussion at Google Webmaster Help.