They are allowed to look at you and take notes and sell them.

Stalking is considered illegal in most states and countries.

Frankly I can't figure out why stalking a single person is illegal but stalking a billion people is considered good business.

I understand stalking involves more than taking notes and selling them though. The other person has to feel threatened or such. Now maybe you can/make the case that you feel threatened by Facebook, and maybe you can sue them individually (good luck), but I doubt you can make the case most people feel this way.

> The other person has to feel threatened or such.

Nope, they just have to want it to stop :)

Depending on the specifics they may not be. I live in a Condo tower and my mailbox isn't visible from the street, so if you decided to take notes on me as I read my mail you'd be trespassing.

The specific scenario isn't the point - but the fact that a semi-obvious scenario could be incorrect sorta is. Regulations are complex and tech has a terrible history of playing fast and loose with regulations so it's not like an imposition of regulations would be inappropriate or unwarranted - there are good and bad apples, and the bad apples spoil the bunch.

Being in a public area in a private business doesn't afford you that privacy. The trepassing charge would be possible if a security guard asked the person to leave.