Look at the fundamentals:
Facebook has a shit-ton of information about you already. It knows your social graph, it knows what you "like", it knows your name, it knows your relationship and job status, it knows your religion, it knows your age, it knows your education, it knows your phone number and address. These are things you may have explicitly told it. It now also knows a significant fraction of your browsing history. Web-savvy folks with a solid business sense know how that sort of information can be easily monetized.
Consider what Facebook can easily determine from the information it explicitly knows: your income, debt level, and credit rating; your family size (and stage); your job satisfaction; your relationship satisfaction; your hobbies; your fetishes; every single one of your buying patterns. Facebook can not only determine a better set of ads to present to you on facebook.com it can resell information elsewhere to others so they can present better and more effective ads/deals on potentially every site you visit and offers you receive in the mail and through other mechanisms as well.
A hypothetical example: facebook is able to determine that you are a gainfully employed person with a much higher than average income and no children, they can also tell that you "like" a lot of things about motorcycles, you visit motorcycle news sites and web forums several times a week, you visit motorcycle manufacturer pages often. How valuable is that information to a local motorcycle dealership?