1. Because if you went to an event but didn't post a photo of it, were you really there?
2. To show that you can cook an average-looking breakfast.
3. To show your real birthday to make sure no one forgets.
4. To show you hit the most touristy spot in every city.
5. To tag your location
6. To post your ultrasound on Facebook before you even show your mom.
7. To prove you have a life beyond House of Cards and True Detective.
8. To brag.
9. For validation and recognition.
Statistic Source: http://www.mdgadvertising.com/blog/the-sad-state-of-social-media-privacy-infographic/