Based on my experience working with a number of companies (n > 15) on onboarding flows, Email + Facebook signup (on the same form) is the most effective. This is trailed only slightly by offering only email signup. Most companies that offer Email + Facebook (on the same form) see a 70/30 split favoring email.

I don't understand why any developer would use FB anonymous login when you can just use email which is equally as effective.