Some apps and websites allow you to log in using just an email address or phone number thanks to Facebook's Account Kit SDK. Until now, two-factor codes for mobile logins were sent via SMS only, but an update is adding WhatsApp verification.
Since WhatsApp is so popular in certain parts of the world — Asia and Europe, especially — it makes sense to offer this option. Many people only use SMS for verification codes, with all their conversations with friends and family going through WhatsApp.
Developers using Account Kit for their app or website login can now disable SMS as a two-factor option if they so wish, although that might be a little restrictive depending on the region. For more about Account Kit, see the Facebook Developers website.