can not get email address from your social account facebook

Just a guess...

The facebook documentation says:

note: this field will not be returned if no valid email address is available 
for the user

Source: https://developers.facebook.com/docs/reference/api/user/

So when an email address become invalid (users who haven't used facebook in 
years and have destroyed/changed their email address since for example), 
facebook may stop returning this field.

5% of invalid address may sounds big, but if I check my facebook friends, I 
think more than 5% of them never use their account, and I wouldn't be surprised
if the address they used when they signed in is now invalid.