I was checking out on a website recently and once I placed my order this (below pic) came up and I'd love to figure out how to do this on my store too. Once the purchase is completed, the customer has the option to share on Facebook to get something free with the order they just placed in return for sharing. Any help on if there is an app for this is appreciated! Thanks!
I'm pretty sure there are many discount apps on Shopify that can do what you want.