It depends on the security needs of the service. Email addresses are discoverable making it easier for an attacker to target a particular account while username

It depends on the security needs of the service. Email addresses are discoverable making it easier for an attacker to target a particular account while usernames give the user the opportunity to further insulate themselves from being discovered on a service and potentially brute-forced or cross-ite attacked. Perhaps less important for a social sharing service and more important for a financial service, but it depends on the user.