Login with social Media i.e Facebook is very professional thing which makes your app very touchy
How I will achieve this job:
1. Create App
2. Integrate the App with your site.
3. Do all App Settings
4. Live the App
How social media login or signup works: Consider a user comes on your app and you provide the facility of login or signup using any social media. When a user click on button he will be redirected to that social media and user will be asked for login information to enter. User will enter the information and the app will ask him permission. If user permits it, then he is redirected to your site. Your website will fetch all the information you needed.