@Rosemary12 Do you have Verified ID turned on as a reservation requirement? The lack of Facebook activity wouldn't bother me. Many people set up FB accounts and then never use them. Their domicile may have changed since they set up their FB page a few years ago.
I would ask more about their plans during their stay to make sure your place is the right fit. Get the guest to talk with you. If you still feel uncomfortable, you can decline the request.
Lack of reviews or being new to Airbnb in and of itself would not bother me. I've hosted guests with no reviews and they have been really great.
Go with your gut feeling!