In the case of Facebook, you either put this in your profile, or the kinds of pages and events that you like and interact with will give this away.

Not saying that it's not creepy, just that with the way we use the internet these days, it's not an epic feat for an advertiser to figure this out given the amount of data we put out there.

Actually in this case it's pretty easy. If you're gay and use dating apps, and didn't opt out of application usage history in your settings, the presence of those apps in your history will flag you as having that interest to advertisers. The google analytics get resold/repackaged, either directly or indirectly through data brokers, to Facebook. Facebook suddenly starts displaying ads for expensive mens' underwear.

Ha, I get ads on FB for expensive men's underwear. Their algorithms clearly have room for improvement.

Really? Google Analytics repackages and sells user data to third party data brokers?

> Does that mean that so many advertisers track your activities so thoroughly that they figured out you're gay?

There was a thing a few years back where Target was able to determine if someone was pregnant based on their purchasing habits [0].

Facebook, Google and others gather and track orders of magnitude more information than Target could ever dream of can certainly figure out if you are gay, straight, and anything else.

0: https://www.nytimes.com/2012/02/19/magazine/shopping-habits....