Rather than asking him the question of why in a simple rudimentary form, give the guy motivation to quit facebook instead. Facebook employs guilt tactics about people they know to get them to stay, and if they quit, they're likely to go back because of in person connections. By sounding condescending, the person you're replying to is likely to justify staying rather than quitting.

i have an account from when i was a teenager that i sometimes use to talk to family. why do you ask? based on my anecdote we are all tracked by facebook, whether or not we are on facebook.