I used to log out of Facebook once I finished with the site because I realized they could track me with all the “Like” buttons on plenty of pages not on Facebook. But then I realized that was stupid, they can track me anyway through existing cookies, and even if I were to delete all Facebook cookies after logging out they could still track me via IP (Internet Protocol) address (not that they do—in fact, I don't know if they do or not, but they could!).
I still log out though. Cargo cultish [1] I know, but it makes me feel better, even when intellectually, I know it doesn't do a thing.
Which is why I found this ad [2] so disturbing. Imagine that last line reading “we just did … and no, I don't mean for likes.”
[1] http://neurotheory.columbia.edu/~ken/cargo_cult.html