I do understand. Online advertising in a free product is an essential part of the business model where I work. I do understand perfectly the nuance of which you speak because I deal with personally identifiable information everyday.

Where I draw the line is when Facebook goes out of their way to learn about their users as a way to enrich their advertising platform through sneaky means. For instance, every 'Like' button on any web page can track where you go on the internet. While they don't hand this information directly over to their advertisers, its essentially spying without consent and then profiting from it. I consider my browsing history personal data because its mine, not Facebook's.

Would it make you more comfortable to say "they spy on me and then use that information to target me in ways I did not want Facebook's clients to leverage?" This holds especially true since that tracked behavior happens outside of their walled garden.

I say "sell your personal data to advertisers" because colloquially more people understand that and its a recognizable sound bite. But the nuance is there, I deal with it everyday in my job so I understand it, and I think Facebook crosses the line.