If full name is not really important, i suggest to ignore it.

Username / Password / Email

One step further is to make email as the username. This removes a field from the form and also improves the chances that the email given is really used.

And some sites just use Email instead of Username :)

If you use our registration tool, most of your form will already be filled out for people logged into Facebook.

http://developers.facebook.com/blog/post/440