Setting facebook itself, and the OP's specific decision aside for the moment...
> Who or what defines if a company is ethical or moral enough? You?"
> I certainly wouldn’t pretend like I am on some moral pedestal and hand out advice to young grads to skip taking a fantastic opportunity because it conflicts with my biases.
This is like saying that that ethics is equivalent to a category of personal biases, that they are therefore arbitrary and completely personal, and that therefore nobody has any right to use their judgment about the ethical content of the actions of other people.
"Are banks more ethical? (...)"
This is like qualifying the above by saying that 'not only is everything relative, but let us look at things relative to these other notorious actors. In that light, who indeed could condemn a person for doing X?'
Although the contents are drastically different, this sort of argument is logically similar in form to 80-year old nonsense such as: "if all the other restaurants are forbidding black people from entering their premises, who are you to say they are wrong?"
While I personally think that there is an ethical way to work for an unethical company, I don't think that pretending there are no issues to carefully consider is an adult behaviour. Its like an hiding your head under the blankets when something unpleasant is happening.
...in other ethics related news, a UN report has recently declared that 1 million species are in danger of extinction. Will we say that concern for this also is a relative, personal bias?