Possible reason for the Discrepancy
Facebook Ads and Shopify use different tracking methods. Facebook counts a link click when someone clicks on your ad, regardless of whether they reach your store due to various reasons (slow loading, redirects, etc.). Shopify registers a session only when someone lands on your store.
Make sure your Facebook ad URLs are tagged with UTM parameters (e.g., utm_source=facebook&utm_campaign=summer_sale). These parameters help identify traffic specifically from Facebook ads in Shopify analytics.