I currently have a Fitbit Activity Tracker and is looking to switch to the Apple Watch. My question is, is there an app out there where I can continue challenging my Fitbit Friends with the Apple Watch?

Hi

Apple Watch supports Activity Sharing with other Apple Watch users, but not with users of other fitness tracking devices:

Share your Activity with your Apple Watch - Apple Support

You may find that the following third-party app meets your needs for challenging your friends:

activity.club

• "Challenge your friends! You can directly challenge friends in activities such as step tracking and runs to bicycle distance challenges and more."
• "AC supports Apple Watch, Fitbit, Jawbone, Garmin, Misfit, Withings, Microsoft Band, Under Armour and more as well as fitness applications such as Apple Health App, Runkeeper, Strava and Google Fit."
• App: https://itunes.apple.com/us/app/activity.club/id903459923?mt=8