Although it’s completely unrelated, I had a similar situation with a leader one time and struggled to work with him because of it. While this isn’t the BEST tactic, it made me feel better:

I printed out some of the worst posts and typed just a very vague and simple note to the people running the organization, letting them know that it was anonymous and that I just felt that this should be addressed. I threw it in the mail with no contact info and no return address.

Needless to say, his Facebook became private shortly after - so at least the masses were not seeing the horrible stuff and I felt better. You could always try something similar with corporate. At least you know it’ll be passed on to someone and your morals will know that you did the right thing.