Sticking to the "net positive / net negative" terms, imagine Facebook contributes a number of positive / negative points to the world. Now the sum can be a net negative, but that doesn't mean some line items aren't positives.

Where do you work? Is your company flawless? Are you a contributor to every single one of its misdeeds merely because you work there? This isn't an easy thing to answer for everyone. I think it's certainly a question more Facebook employees should ask themselves though.