BuddyPress.org

Ah, okay, SOLVED.

I was logged out in that screenshot but Safari must have been holding a cookie identifying me a member all the same, resulting in the username field, password field and submit button appearing but not the Facebook button.

When I finally realized what was happening, I tried with a different browser and, hooray!, the Facebook button was right there.

So, now that I can see it, I would just like to say, officially, WOW, amazing feature, this is going to be fearsomely popular with users.