1. I had come to believe FB couldn't be trusted with my data.
2. The more friends I gained the worse the experience became. It went from people showing their vacation photos, which I was ok with, to a stream of meaningless nonsense that I didn't want to see. I.e Yeah, OK, you had a coke at lunch, but should I really care?
3. It became an unpleasant chore having to keep on top of even the valid activity.
It's worth noting that in the last year of use I just put a status up declaring that I no longer actively used Facebook so please contact via email instead, but the distrust of Facebook grew to a point where I ultimately decided to delete my account.
I'll never go back, and I hear they're full of ads now.