One key success metric to measure the success of Facebook Shops could be the conversion rate. This metric would measure the percentage of users who actually make a purchase after visiting a Facebook Shop. A high conversion rate would indicate that Facebook Shops are effective in converting browsing users into paying customers. However, it's important to also consider other metrics such as customer retention, average order value, and customer lifetime value. These metrics would provide a more comprehensive understanding of the success of Facebook Shops and the impact they have on the overall business goals of the company. Additionally, it's important to continuously collect user feedback and monitor user behavior to make data-driven decisions and iterate on the Facebook Shops experience.