Hello Adam,
I think this app fits your needs: https://apps.shopify.com/checkout-boost
It allows you to set free gifts or discount for customers, depending on product type or cart value of your choice. Customers will need to share their cart on Facebook, Twitter,... to receive free gift or discount, which not only encourages them to checkout (to catch your offer), but it also helps you reach more new customers.
Hope it helps :)
