I've written about the benefits of using Facebook connect as a registration path: 1. it’s becoming universal and therefor familiar 2. users are more likely to pass information through via Facebook than to a new site 3. it expedites the flows and therefore improves conversions 4. it about more than efficiency. For instance: you can add the facepile for conversions, mobile flows can occur with a phone number (rather than an email address), etc 5. it expedites and simplifies mobile logins (both applications and mobile web)

Here is yet another example: I often stare at login screens and wonder which email / password combination is right. In this example (Groupon), I entered the wrong email / username, and it auto-corrected me based on my Facebook information: