I had it happen it to me as well, but the only link I can see was that I was texting with that person for the first time that day. And then I saved the number a

I had it happen it to me as well, but the only link I can see was that I was texting with that person for the first time that day. And then I saved the number as a contact in my phone. Then I guess WhatsApp picked up that change and sent the data back to the Facebook servers. An hour later, the person showed up under Suggested Friends.