IMHO this mostly depends on *personal priorities*, whether you/ the user prefers to use your username or your email address to login to a website.

For a project I just implemented the option to use either your username ***or*** your email address to login – and I suppose that nowadays actually many websites allow to use either or. 

[edit] Some additional thought: I just checked *xing.com* and *linkedin.com* and *plus.google.com* and *ello.co* and *pinterest.com* – but they all ask for the email address. At least they suggest to use the email address – maybe it would also be possible to login entering the username. So maybe it actually **is** a bit more user friendly for both *user* ***and*** *site admin* in case somebody forgets his/her password.

In this quick round-look I could only find *twitter.com* offering login through *email address* or *username* or *phone number*.