Data that shows users gained by offering a single sign on option vs. not offering it?

It would be nice to see if there is any available data about SSO usage by vendor and whether offering SSO equates to a gain in user registrations.

Anecdotally, I've noticed general a decline in use of single sign on for Facebook in the past few years. I would guess that has something to do with their trust problem.

Any insight is appreciated. Thanks :)