3 Facts :
(1) The middle user stays less than 5 minutes on a website.
(2) most users leave when they see a registration form with more than 3 fields (login/password/email)
(3) more than half of internet users have a facebook account
3 answers :
(1) I will provide you with 2 plugin that will allow you to add facebook connect to your website in less than 5 minutes.
(2) I will provide full documentation and support to create your facebook account and/or app easily
(3) Your valuable users will register faster and will never forget your website anymore.