Strava like: FitTrackee and Endurain are planning to implement ActivityPub so yah there's gonna to be one
IMDB like: NeoDB and LibRate
Not sure about the rest
gift economy/trading platform (e.g. like freecycle)
buying/selling (e.g. like ebay)
local community/bioregionalism networks (e.g. what nextdoor should be)
These seem kind of ideal for a federated network, IMO.
I actually think Lemmy would be a pretty decent format for something stackoverflow like - just maybe needs to UI tweaks to minimise the visual space that replies take up, plus maybe answered post flair
It's been around for many years now and famously was used in the consultations for the constitution of Iceland. We also used it in Bulgaria back in 2013 and had a community of more than 3000 users, but it lost traction due to being ignored by politicians and controversial debates.