How would you like a potential employer to look at your Facebook "Likes" and conclude that you are not smart enough for the job? Or, equally bad, that your sexuality is incompatible with the employer's views? But, you say, nothing in your Likes speaks to these issues. So you might think, but a joint study between Cambridge University and Microsoft Research predicted a large group of Facebook users' IQ's, sexual orientation, and other traits to a high degree of accuracy using their Facebook Likes. Here is a link to an article on the study:
Not convinced of the method? It really doesn't matter. These studies are being done, they will be found on the internet (or otherwise) by employers, and quite probably incorporated into hiring decisions whether they are accurate or not.
As this type of use is made of Facebook data, more people will have to ask themselves whether they can afford to be on Facebook. And that is not a question that will be good for Facebook's business.