My personal favourites are web forums (they offer threading, usable archives, search interfaces, RSS-feeds, the option to give different "rights" to different people, direct messaging, ... ).
I haven't subscribed to a mailing list for ages, and I'm reading most of the mailing lists I'm subscribed to only about once a week.
These days I get most interaction on G+ (which does have "communities"), but that may be due more to the people interacting than to G+.
Best
Heinrich