Say I want to show an "ad" to white, heterosexual, protestant men with libertarian leanings, no history of depression, unlikely to use alcohol in their free time, who don't look pornography, and are predisposed towards compliance with authority currently employed in the technology industry earning over $150,000. Say my "ad" is for a high-paying job opportunity. Or housing. Facebook definitely has the ability to let me do this.
I find that unsettling, in that people can use Facebook to apply prejudices against information I'd prefer to keep hidden but that Facebook has inferred via data-mining even without knowing who I am or actually learning that information about me.
The "stalker" scenario presented here is certainly frightening (although as a heterosexual man I can only really understand the fear in the abstract), but the "reveals information I'd prefer to keep hidden to people with a financial motive for knowing it" scenario is already getting uncomfortable.