It's not like we didn't know Facebook collects data on us. It's all right there in the privacy agreements that almost none of us bother to read. However, when it comes to their ads, they've got us pretty well figured out.
Ever look for something on Amazon only to see an ad for that very same item on FB almost right away? That always creeps me out, but that's not the extent of their advertising efforts. FB generally knows most of the important parts of your life and advertises to you accordingly. They collect info on your family, work, political leanings, and interests without you really needing to provide any info directly.
For example, I don't list my kids as family members on FB, but they know that I have a child between 1-2 and another between 3-5 simply because I post pictures of them! This just highlights a small portion of the info they collect about you to help them advertise. You can look at (and change) the info they use for you by following these steps:
1. Go to Settings
2. Click on Ads
3. Click on Your Information
4. Under Your Information, Click on Your Categories
Turns out Big Brother's name is Mark...