Three possible approaches, from most to least verbose:

1. http://sloblog.io/login has a nice, explanatory landing page.

2. https://www.voo.st/ has a small string of explanatory text at the point where a user chooses between Facebook or Persona auth.

3. http://crossword.thetimes.co.uk/ has a simple, unbranded "log in" button that just opens the popup.