code to get social media usernames if visitor is logged in to a social account

I would like somebody to write some code so that if a person visits my website and is logged in to a social media account we can show them the relevant social media buttons. This would be even better if we could greet them with there user name or part fill a registration form with their username. I am not sure if this can be done, i know there is code which will show you if somebody is loggedin to their facebook, twitter or google and i know this can also be done using some social api's. In your bid please explain what you believe is possible and a brief summary on how you will achieve this to demonstrate you have understood the project. If you dont provide this information your bid will be ignored. Thanks