Recognize users redirected from social networks shared content

I've been researching a lot and couldn't find anything close to my story. I'm stuck currently.

I need to figure out whether user comes to my website by clicking on shared content from social networks (Facebook, Twitter,Linkedin ) or user just accesses directly?

For instance, I shared my website on Facebook and some of my friends see it and click on the share content, so I want to recognize those users as user that are coming from social virality.

Any idea how it can be implemented ?