Hm, that's a tricky situation. If possible, it's best if you convey the idea that your new friend is not alone in the boat. Since you said Facebook ID is fine, my personal recommendation would be something along the lines of,
Ah, I don't usually share my whatsapp credentials publicly. Would my Facebook account work instead?
The advantages of this approach are twofold:
You make it clear that this is the way you treat everyone, making them not feel "left out."
You leave the door open for them to try another option.