I use my own SDK for iOS and Android Facebook in my application. In general, I get a list of friends, each with a name, birthday (if it's open) and a username. As for the property of a birthday, I know that it can be forbidden to appear in mobile applications (or in general). But I see that in some rare cases the username is NULL. I could not find any explanation in the documentation on Facebook.
Any help would be great.
Some people simply did not select a username.
www, facebook.com/profile.php?id=user_id, facebook.com/username.