Why should I care if a corporation is gathering stores of information about me via the internet?

Google, Facebook, Microsoft—all of them have been accused of amassing vast amounts of information about the people who use their products. It is often treated as a privacy disaster. Why, though, should I care?

I understand that the information is valuable to them when compiled, but it’s not like I could demand a large sum for my individual “contribution” to the database. Besides, I get things in return for my data in the form of free services.

Nor is my personal privacy very much threatened. My name is no more distinct than a string of numbers—and probably less uniquely identifying—as far as Google et al. are concerned. My grocery store is more likely to pick me out from its list of rewards club members than Facebook is to pick me out from its list of users.

What am I missing?