I'm a few years late on getting into this conversation, but I wonder if the original poster's professor was from the U.K. too? In my first year of teaching in higher education in the U.S. I'd correct "societal" to "social" in papers because I thought it was just a common mistake (of which there are plenty in student writing, after all). Finally I saw it so much I concluded that it was accepted usage, right or not, and stopped correcting it. Then today, seven years on, I saw it in the New York Times and had to admit that it might even be official usage - hence my Google search to try and work out why I'd never come across it in the U.K.
I can't find much on it, but it was interesting to read the various comments above. I'd note that just because it appears in U.K. dictionaries doesn't mean it's in normal usage in British English. I certainly never saw it in a newspaper or heard it on the radio - though that's not to say that there aren't some academic fields in which it gets used.