May I ask what plan you're currently on? If you're not on the Shopify Plus plan... then you won't be able to customize your checkout page.
Have you tried looking for a Shopify application that asks for the customer's social media account? Maybe it already exists...
I also found this which you might find useful: https://community.shopify.com/c/Shopify-Design/Cart-Use-cart-attributes-to-collect-more-information/.... Again you could only do this if you were on a Shopify Plus account and even this requires custom work from a Shopify developer.
I hope this answers your question,