I think the bigger point has nothing to do with social buttons or login UX.
Test your changes independently, and make incremental changes
They thought social buttons improved login success. They didn't. An unconnected copy change improved login success. If you test these things independently, you'll get much better insight into what makes a difference.
> Test your changes independently, and make incremental changes
That was my take-away as well. I'm prone to accumulating a list of changes that I'd like to make to my site and then, when change fever strikes, I do them all in unison. When something goes wrong (or right), it's impossible to tell which change had what effect. It's a hard habit to break.
Test your changes independently, and make incremental changes
They thought social buttons improved login success. They didn't. An unconnected copy change improved login success. If you test these things independently, you'll get much better insight into what makes a difference.