Sadly I don't think these devices fit within SuperUser's scope.

As it is more an IoT device you might want to try [Internet of Things](https://iot.stackexchange.com/). According to their [help files](https://iot.stackexchange.com/help/on-topic)

> If you have a question about...
>
> - controlling, automating and sensing the environment using electronics ('smart devices'),
>
> ...
>
> ... you're in the right place!

I would say that a FitBit is a smart device that senses it's environment. You might want to check on their meta first though.