I do know that Fitbit needs BT 4.0 Smart, which apparently will be made available to Lumia after the next update (Black or GDR3) - I stand to correction on this - so once this is available we may start seeing an updating of applicable apps.
You could also look at this app - active fitness - it also measures activity, but I uninstalled it after a few days. It is similar to Fitbit, but not quite the same standard.
You could also head on over to DC Rainmaker, he has all the latest info on new health trackers and devices, unfortunately, there are a lot of new devices coming up which are not supporting WP.