Contact information should not be an identifier. It's like when a B&M store uses your phone number to ID you. I don't want them calling me and I don't want your website to email me.

The URL is simply a pointer to my identity, which is all I should have to give. The different formats the providers use is what makes it confusing (example.com, username.example.com, example.com/username, and now username@example.com)