However it might be done, logging in and signing up are two different things. The sign up path is almost certainly more complex than the login path, regardless of whether social login or a more traditional login are used.
These crucial differences are not negated just because the technology allows the process to be done in one click.
For example, you may want to do / get your user to do the following on sign up that you wouldn't do on login:
- agree to your sites terms and conditions
- enter one off data, such as a date of birth, or consent to your using of such data
- give them a sign up bonus
- redirect them to incomplete profile elements
- pick an image, perhaps from their available social media image uploads, this they might like to repeat again later, but it's not something that would occur on every login
Social signups also give you access to your users social data, which you can't just take without explicit consent - we are all familiar with the 'this app would like access to your x, y and z do you agree?'. You might be able to take the data without them entering it, but you cannot do this (I am not a lawyer, but I have a feeling this would actually be illegal wihtout consent).
Login on the other hand, if done with social accounts, should be one click and in (assuming they are signed into their account of course). As smooth as possible for the user.
Stack Exchange is a perfect example of this. Clicking on a new Q&A site prompts you with a set of social logins, choosing one of which leads you to a reminder that they will use your email address for communication, where as login with a social login just clicks you straight in.
Having said all this, it's perfectly reasonable to have one button: 'connect with facebook' for example; and have your code notice if this is a first time login and follow the sign up path but from the users perspective these paths must be kept distinct.