The problem is nothing is editable (full name, birthday, location, email -- ok, they can have gender). If I'm signing up for some brand-new, unknown service, I'm usually not inclined to divulge anything more than an email address with an alias (+newsite@gmail.com).
You might want try out our service (Spockly.com) to tackle this problem: it allows you to limit your sign up form to email (& password) and get the basic info like age, gender, location from us. So sign up is straightforward, but you still get some insight about your users. Of course, it only works with people who have public profiles on social networks.
