I have no problem with mailman. If it had a way to search the archives that would be good. I also think we need some sort of place to upload code and documents. So groups.io would get my vote if we were going to do something external/not hosted ourselves. I don't see Facebook as a good place to really do anything productive.
By the way one week has gone by on the ET Docket 17-215 (its open until October 30 for comment), and we have three comments. (https://www.fcc.gov/ecfs/)