iOS: We’ve covered some of the apps that send data to iOS 8’s Healthkit. Fitbit doesn’t directly talk to Healthkit. Sync Solver fixes that problem.
After you install the app, it syncs in the background once a day to Healthkit. Fitbit's API doesn't allow continuously syncing like the Fitbit app does. If you have an iPhone 5S or higher you'll need to tweak Healthkit to prevent recording steps twice.