Why don't you wait until the first time that they user wants to post something before asking for Facebook details. I'm sure you are completely honest, and would

Why don't you wait until the first time that they user wants to post something before asking for Facebook details. I'm sure you are completely honest, and wouldn't abuse the permissions you've been granted. But I don't like giving out any more personal details (phone #/email address/Facebook profile) than I absolutely have to. But then I guess I'm not really your target audience either.