At first, it was believed by many that it was due to a politician:
http://speier.house.gov/images/Corre...es8may2013.pdf
However, it's turned out that some people have taken it upon themselves to report the sites for posting nudity and pornography. One person doing the reporting was a military wife who found pictures of women in bikinis and such offensive, and there was an entire site which prides itself on being a bunch of POGs that got in on it. The "Pogworks" site has claimed responsibility and is going through and reporting the newly created replacement pages.
What I'm gathering is that the reporting would have gone fairly unnoticed until Congresswoman Speier came out with her statements whereas now Facebook is taking these reports by users much more seriously.