Making an answer by elaborating my comment:
Have you ever heard of Jodel? It is a mobile app that may suit your needs: the base principle is to post anonymously on everyday things using everyday language.
About your criteria:
it must have tons of funny and irreverent contents, and back-and-forth exchanges
Have a look for yourself, I find the content pretty random. Besides, the expression is really freed due to anonymity.
full of colloquial usage, swear words, rudeness and banter
It is mostly used by students. Living myself as a foreigner in Germany, I found many words and expressions which I had never understood before, that I could learn and successfully re-use in everyday life.
It is city-based so you can instantly have access to different "slangs" of different regions just by changing your settings!