I have seen this in a few sites I run. One is a business site, another is a special interest with specific demographics, and the third is a blog.
It all started with Google shuffling their algorithms, with Panda then Pengiun.
I saw traffic drop on all three sites. Some coninciding with Panda, and the other coninciding with Pengiun.
One site was the top site for certain search terms for many long years. Not anymore. That site saw a 7.5X drop in pageviews per month traffic. Another site saw 3.5X drop, and the third was 2.5X.
What is weird is that Google de-indexed one site because of "un-natural links". When I contacted them, I asked what the links are, so I can remove them. They never came back with any definitive information, and sent the same template email saying site de-indexed because of un-natural links. It took 3 or 4 tries, and then they reinstated the site back in the index. They never told me what the links are, and never explained why they de-listed the site nor why they reinstated it.
Another thing of note: some sites no longer show up in Google searches. For example, here in Canada we have a restaurant review site called Restaurantica. It used to show up in the first few searches for restaurants in the area (Southern Ontario). Now, I don't see it at all on the front page. Seems Google decided that Trip Advisor and Urban Spoon are the authoratitive ones for restaurant, and Restaurantica is third class or something.
I also noticed that the search quality for Google has gone downhill starting in 2011. Really stupid matching of terms, some partial strings even. I've never seen Google's search that bad before.
They are for sure dumbing things down, a general trend in the industry in the name of "user experience" and such. You see this in Firefox with the dumbed down Australis, which requires Classic Theme Restorer to undo some of the damage.