That was good-enough anonymity for most people, it seems, although some entered specific nicknames.
A few days ago, the Fitbit user groups suddenly starting displaying most people's full names, eg. "Eric The Half A Bee".
Not everyone in the U.S. is happy about that:
https://community.fitbit.com/t5/Fitbit-com-Dashboard/Full-names-are-now-showing-for-friends/m-p/1836501#M63452
and it may be a problem for the NZ Privacy Commissioner too?