if signups are your metric, then I would guess you'd see an increase.

however, as others have mentioned on HN, by reducing friction you increase the amount of "tyre-kickers" - people who just want to check things out and leave.

that's not such a bad thing if your cost-per-user is negligible, or you have some kind of post-signup campaign in place (e.g. an email after inactivity that tries to "reactivate" the user).

in those situations, the more people you can expose your company to, the better. I'd wager that many startups don't implement Facebook signin because of the engineering overhead that could be spent on sexy new features.