Here's the deal. Facebook set up an arrangement that allowed its users to vote on policy changes, with the vote only counting if enough people participated. Then Facebook announced that it was breaking its own rules and removing the voting system, as well as making other privacy changes. When enough people pointed out that this was a bad idea, Facebook backed down and let the users vote on the policy changes. I don't know what happened after that, because I stopped following the story since I left Facebook before then.
First, I never knew that I could vote on policy changes on Facebook, and I am the type of person who keeps track of that type of opportunity. The first time I heard about the voting system was the email from Facebook telling me that the voting system was going away. If Facebook has the resources and wherewithal to send out an email explaining the demise of the voting system, why didn't it let the users know about it beforehand?
Second, if Facebook randomly breaks its own rules at will, then what guarantee do I have that Facebook will follow the rules the rest of the time? The selling of people's information is a billion dollar business, which seems to me to be more than enough incentive for Facebook to ignore its privacy policies. Before I had relative confidence that Facebook wouldn't do that. Now...
Third, Facebook continuously changes its privacy policies. While I expect a certain amount of turbulence, the constant changes make me wonder what Facebook is doing. I know that I am reading nefarious purposes here, but so be it.
So those are the three reasons I decided it was time to leave. The advent of Google Plus as a viable alternative also contributed to my decision, though I still would have left after Facebook attempted to break its own rules.