One of the reasons online advertisers like Facebook as an Ad Platform is because of the depth of demographic and psychographic targeting available. You really can get pretty specific about the segment of people you want your page/post/ad shown to.
A group of researchers built a personality modeler based on a FB user's likes.
The big thing advertising online is lateral targeting -- finding connections between disparate interests and possible products/events/activities to promote. Needing to analyze only 100 likes in order to predict basic traits and possible cross-interests is very, very cheap for these analytics engines.
Moral of the story -- your likes define who you are online and shape the ads/offers you see.