Most locals want to know about how to get in touch with raids groups or how to chat with others (to exchange info on rare spawns, nests, or good Research rewards) on platforms like Facebook.
Niantic is investing more in their companion Campfire app, so download it and join the Oahu POGO! group
Most of Oahu uses Messenger group chats to organize raids; there's 1-5 chats per neighborhood/area.
Some parts of Oahu do use Discord (and Discord can be a good way to list good research PokeStops). (There's also an active, separate Laie/Kahuku Discord---by BYUH and the Polynesian Cultural Center).
Facebook groups (these're called "Hawaii," but most seem Oahu-centric)
Pokemon Go Hawaii: ALOHA Region: Well moderated, helps maintain list of raid chats
Pokemon Go Hawaii: Tries to keep posts about Research quests orderly
Pokemon Go Hawaii (8000+ members)
Pokemon Go : HAWAII : A Public group
Pokemon GO! HI : A Public group
The west side uses Discord.
Pokémon GO West Hawaii (300+ members)
Lahaina/west side of Maui uses LINE. Other parts (Kihei, Kahului) use Viber (possible start of a Discord server).
Pokemon GO! Maui (900+ members)
Pokemon Go Maui (500+ members)
Kauai has a Messenger group.
Facebook:
Pokemon Go Explorers (Kauai) (700+ members)