The ability to sign in to WHMCS using social network profiles was first introduced in WHMCS 7.3. This future makes it easer for new customers to create an account and login, it simplifies the process and make it more convenient, plus it is mobile optimized. Currently three social media are supported: Facebook, Google+ and Twitter and that covers the majority fo audience. For each social network you will need to setup an application to be able to retrieve API credentials. That could be done on there developers websites. In WHMCS side login to admin area and navigate to Setup then Sign-In Integrations to find setting for each social network. There is a good step by step guide on WHMCS documentation site. Contact Web Admin if you have questions and need help with your WHMCS website support, upgrade or customization.