What were the alternatives? Requests like that are a huge turn-off for me. Ask me for my email, sure. Ask for the ability to post spam on my wall and I've just closed the tab. It may convert alright, but it also drives away potential customers. Why not give me the option to subscribe by email?
I'm guessing it's because you're likely not their target audience. A lot of people are more than happy to let whoever asks have permission to spam their Facebook feed with free advertising.
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.
