The fully remote teams that I have been a part of all started a tradition of leaving a persistent ${VIDEOCONFERENCE} running that we can just hang out in. It isn't perfect, but is enough to retain the sense of community and support from an office.

I'd love to see a dedicated tool that does "virtual office hangouts" well, where you can spin up rooms, share screens/files/text, easily drop in and out, and see where people are. There are a few out there that come close, but I haven't seen any that let you browse to see various groups/individuals to match walking the halls.