I lost my Facebook account when I recently activated a new iPhone

If you're having trouble logging into your Facebook account, here are some things you can try. Go to facebook.com/login/identify and follow the instructions. Make sure to use a computer or mobile phone that you have previously used to log into your Facebook account. Search for the account you want to recover.

I can't log in to Facebook. | Facebook Help Center

Even you may see this --> Fix a Problem | Facebook Help Center

Still not solved then --> Report a Login Issue | Facebook

Facebook is not an Apple product. Contact the app developer here --> App Support or Facebook Help Center | Facebook

There is no link between iPhone activation and a Facebook account.