I have friends in France who are highly educated and write french beautifully, and yet, for all they could, they do not speak french in a 'pure' form, whether they ever do, ie at work, I have no idea. As they use slang words like everyone else when I see them.
I admit since we have left and the french language has moved on, sometimes I simply do not know what some odd word or expression means and have to ask. If I was there, I daresay I would have just picked it up in passing. And I suppose that was the good thing about us only having french tv for so many years, that I could not only hear some of the argot, but I suppose see it in context.
So how good are you with french slang and do you use it???