Facebook began purging massive numbers of Likes that were believed to be from fake and fraudulent sources. According to the Facebook Security Page "on average, less than 1% of Likes on any given Page will be
removed, providing they and their affiliates have been abiding by our
terms. These newly improved automated efforts will remove those Likes
gained by malware, compromised accounts, deceived users, or purchased
Further they state that "users will continue to connect to the Pages and Profiles they authentically want to subscribe to, and Pages will have a more accurate measurement of fan count and demographics. This improvement will allow Pages to produce ever more relevant and interesting content, and brands will see an increase in the true engagement around their content."
As you can see in the image below from PageData, some fan pages have lost a large number of fans.
John "Mike" Wright