We tested this with http://www.reedge.com for a client that integrated the facebook login on the checkout page of a Magento shop. I remember that Around 8% more finished the checkout compared to when we did not show the facebook login option.

The problem was we did not test the actual login, just having the option. The client did not give us details on revenue/turnover or hoe many people actually used login of facebook.

It was a health product on the US market.