Fitbit has no plans yet to use HealthKit and associated Health app for iOS 8. This was told by an employee of the company on the Fitbit forum. At this moment the company sees yet no way HealthKit can improve the user experience.
The reaction of Fitbit will be a disappointment for some iOS 8 users. Integration with HealthKit would make it possible to collect data from Fitbit activity trackers into the Health app. The information would also be available to other apps.
Fitbit, however, didn’t close the door completely: although there are no plans to integrate HealthKit, users are invited to submit suggestions. These suggestions are passed to the Fitbit app development team, so maybe they can still add support.
Competitor Jawbone does use HealthKit in its upcoming app. This allows the app to measure your daily steps, without the need for special bracelet. The M7 chip in the iPhone 5s or M8 chip in the iPhone 6 are required for this. Fitbit has no reason to use specifically HealthKit for this: the Fitbit app does the same for some time, but grabs the movement data in an alternative way.