Google started an algorithm to remove spam out of search results!
Panda 3.5 rolled out!
Google employees officially call this update as "webspam algorithm updated".
If you are trying one among this to rank. Then you might have seen the drop in rankings.
For more info at http://googlewebmastercentral.blogspot.in/2012/04/another-step-to-reward-high-quality.html
A small snippet from official google webmaster blog:
"In the next few days, we’re launching an important algorithm change targeted at webspam. The change will decrease rankings for sites that we believe are violating Google’s existing quality guidelines. "