That's a tough question as to when following becomes stalking, whether you're talking criminal activity or just police surveillance. Think about papparazzi.
Look at People v Jones; that's the US Supreme Court case on police tracking by GPS tracker. Discusses these issues. And that raises the point that since all privacy rights are balanced against other societal rights and obligations, like safety, it matters greatly who is doing the stalking/watching/collecting. People seem to be OK when Google collects certain data, but less happy when that same data is "shared" with the feds. And that's not irrational because the feds can kill you with a drone (but not Google...yet; wait till SkyNet).
The drone issue is fueling a lot of debate on this too. And it's pertinent to us, as photographers. We invade people's privacy all the time. If you don't want Google looking at your photos, OK. But what about those people in those street photos we take? are we invading their privacy? even without putting them online at Google or in a challenge here?
I DO read the privacy disclosures and deliberately choose what to put where. That's why I have email in Canada, for example, and not in the US where I live. And a secure email account in yet another country. Since I live in CA (which has increased privacy protection over and above US law) and near Google it is, I grant you, easier for me to suss out what I should and shouldn't give Google.
I wish Apple were more transparent (on this and many other issues). So I avoid using them even more than Google. But when I am really concerned, I turn to outfits that are explicitly about protecting my stuff. They aren't free, but you get what you pay for.
And I was heartened to see that Apple, Google and others are taking a stand against the US fed's attempt to force the installation of backdoors, sort of ClipperChip v. 2. Good for them.
Meanwhile, if you do upload to Google it is amazing to see what their search can do. Kinda freaky. Like if they can figure out that's a picture of a mountain, they can figure out I like skiing, and then sell that info. I'm OK with that but not everyone is. But it is driving stuff like facial recognition software, which is now incorporated in cameras, so we get a benefit. But it also means that Google or Facebook might find YOU in that photo someone took of you on the street.
TL;DR: be careful out there. With your stuff, and with other's privacy.