It is currently possible to share a membership database between sharepoint 2010 and an asp.net application.
Can I share an asp.net identity database with sharepoint 2013?
It is currently possible to share a membership database between sharepoint 2010 and an asp.net application.
Can I share an asp.net identity database with sharepoint 2013?
yes you can. check this post: http://blogs.technet.com/b/ptsblog/archive/2013/09/20/configuring-sharepoint-2013-forms-based-authentication-with-sqlmembershipprovider.aspx nothing changed in SP 2013
I'm sorry, obviously I wasn't focused enough while reading your question. I didn't try it myself but ACS with custom identity provider may help you: http://social.technet.microsoft.com/wiki/contents/articles/22309.integrating-windows-live-id-google-and-facebook-accounts-with-sharepoint-2013-white-paper.aspx http://blogs.msdn.com/b/mcsuksoldev/archive/2012/11/02/azure-access-control-services-creating-a-custom-identity-provider.aspx