If a visitor needs to sign-on at your site and they don't have any social profiles, their only option is to use a traditional (double opt-in) registration process. That is why websites should always offer those as a fallback.
Offering single sign-on is very easy to implement, and has many advantages when you use a third party hosted service. However implementing the above fallback, (only for the very small percentage of visitors that need it) may be not worth the development time spent.
Is there an easy(developer) and smooth (visitor) way to offer said visitors to sign-on to a neutral social id provider, like ea. openid, and redirecting them back to where they left your site, already signed-in, once they have.