2. Don't force visitors to use a 3rd-party provider. Always have a local option. As a counter point to (1), many visitors won't want to use their Facebook/Google accounts for authenticating to other sites.
I tend to prefer the oauth options myself... I don't want to have to enter a bunch of information, or add yet another site to lastpass if I can avoid it... Though, if they ask for too much, I tend to just deny and move on...
For example, it's interesting that a site within the past couple of minutes to sign into could use a local account, facebook or google... I clicked fb first, they wanted contact info and my full friends list... google, just my email... so I let it in with my google account.
I think it really depends... I wouldn't offer social logins without also having a local option, but it really depends on your audience.
