If you’re on Facebook you’ve probably seen those “It’s been a great year. Thanks for being a part of it.” posts and ads. They’re built up algorithmically from your Facebook activity and if you’ve had a great year they’re a nice reminder of the good things you’ve experienced. But if you haven’t, say you’ve lost a child, they’re a horrible reminder of a year you’d probably like to forget.
Services like Facebook are free because they own so much data about you. You grant them the right to do what they wish with that data. Facebook takes what you post to your timeline and runs it through a program to build these posts. No human is involved. It works just fine for most people, but when you’re not most people it’s a cold reminder of what you give up when join a free (in terms of $) service.