I would be happy with a "27% of participants completed this prompt" or something similar, instead of user-specific stats that could create privacy issues.

That sounds cool. To avoid privacy issues, maybe just an overall completion as the average of all participants

We'd probably need to think about that, especially with privacy levels etc. since if you're private your progress isn't visible from your profile like everyone else's is. Also, could get a little overwhelming for those challenges with 1000s of participants, but then again, in those cases the host may just choose not to look!