We had a big drop in one of our websites starting May 8. Each week since then it seems to have dropped a little bit more.
The site is updated regularly, and never engaged in any kind of link building campaigns. It does get promoted in social media, though. It also has always gotten a lot of links naturally from credible sites. But webmaster tools shows a bunch of sites linking to us that might be spammy, and that we had nothing to do with.
WikiHow comes up often as the first result (sometimes both the first and second result) for some keywords we used to rank in one of the top 2 spots for
Our main topic index pages have just about disappeared from SERPs. The first page of SERPs for various keywords associated with the topic seems to include many more big company brands than they used to.
Interesting, is that when I look at other sites in this niche that are not owned by big businesses, their traffic (at least what's visible in Alexa) have fallen in the same way. So to have other informational sites I know of in other niches that are privately owned and have good content.
Maybe the May 7/8 update should be just called the Big Brand and Wikihow update......