For example, their recent introduction of their "Like" button makes it rather easy for spammers to gain access to my feed and spam my social network. These are just the latest of a series of Keystone Kops mistakes, such as accidentally making users' profiles completely public, or the cross-site scripting hole that took them over two weeks to fix.They either don't care too much about your privacy or don't really have very good engineers, or perhaps both. Facebook makes it incredibly difficult to truly delete your account.
On top of that, account deletion is incredibly (and intentionally) confusing.
When you go to your account settings, you're given an option to your account, which turns out not to be the same thing as deleting it. The so-called Open Graph is named so as to disguise its fundamentally closed nature.
As you'll see, there's no reason to give them the benefit of the doubt.
Essentially, they see their customers as unpaid employees for crowd-sourcing ad-targeting data. Facebook's has a documented history of unethical behavior.