1) I started to lose respect for my friends, after watching their drama on FB. TMI.
2) I was being bombarded with small games that were clearly carved out of psychological tests for the purpose of gleaning data about me. Facebook do not have my authorization for obtaining, storing or sharing psychological profiles, whether or not their fine print says so.
3) I've visited their campus and it is creepy, right out of some dystopian novel. This topic is far too complicated to explain here. I'm sure folks must have written books about this by now.
Read "Get your loved ones off Facebook." [1] and the list of links from Stallman webiste [2]
[1] http://www.salimvirani.com/facebook/
[2] https://stallman.org/facebook.html | https://stallman.org/facebook2.html
I also didn't like their practices as a company, and didn't want to be part of it.
So, I left. I occasionally miss parties--no big deal.
I really, really wanted to like G+, but...