Glenn Greenwald has a good anecdote for this:
"If you think privacy isn't all that important, here's my email, I want you to email me all of your social media, bank account, online shopping, and email logins, so I can go through and learn all about you, your preferences, buying habits, etc. I won't share them with anyone.... No one has taken me up on that offer."
Yet when you log into facebook, or amazon, or anything else, its even worse than emailing someone you've met your logins, because you're giving all of that personal info to a faceless corporation whose incentive is to figure out how to wrangle more profit based on your data.