When Google changed its algorithm it affected about 12% of the searches. The biggest effect was on content farms. Here is an opinion piece from the NY Times on the effect of the change.
You can’t mess with Google forever. In February, the corporation
concocted what it concocts best: an algorithm. The algorithm, called
Panda, affects some 12 percent of searches, and it has — slowly and
imperfectly — been improving things. Just a short time ago, the Web
seemed ungovernable; bad content was driving out good. But Google
asserted itself, and credit is due: Panda represents good
cyber-governance. It has allowed Google to send untrustworthy,
repetitive and unsatisfying content to the back of the class. No more
A’s for cheaters.
At the same time, the goal, according to Amit Singhal and Matt Cutts,
who worked on Panda, is to “provide better rankings for high-quality
sites — sites with original content and information such as research,
in-depth reports, thoughtful analysis and so on.”
Read the rest.