I'm using iOS and Android Facebook native SDK in my app. In general, I'm getting the friends list, each with name, birthday (if public) and username. About the birthday property, I know that one can block it from appearing in mobile apps (or at all). But I see that in some rare cases, username is NULL. I couldn't find any explanation for it in the Facebook documentation.
Any help will be great.
Some people just haven't chosen a username.
If you look on the www page, their profile links will be facebook.com/profile.php?id=user_id rather than facebook.com/username.