Facebook detects if you are logged in Gmail
Not sure how they are doing this, but I have gotten tired of having to play “whack-a-mole” with FB scraping private information from my browser in other ways, so what I have done is sandboxed it: I have a separate “Facebook” account on OS X, and I assume that anything I do on that account is shared with Facebook.I don’t log into Facebook for any reason on my normal user account, and I don’t log into anything else on my Facebook account. They can still sniff certain things using browser fingerprinting and so on, but this seems like the best I can do for the moment on my desktop.