If you have Facebook Like and Share buttons enabled on your storefront (as described in "How to Add Facebook Like and Share Buttons to a Product"), you may occasionally find that one of the buttons fails to appear on a page.
This is caused by Facebook failing to clear their cache for this product page. The remedy for this is as follows:
Enter https://developers.facebook.com/tools/debug in your browser window (or follow the link). This will take you to the Facebook URL Debugger.
Enter the full address for the product (or other page) where the button is failing to display.
This forces Facebook to refresh the Like / Share cache data for this page. Once done, your button should start to appear again on this page. Unfortunately, Facebook does not provide a method to clear the cache for an entire domain, so individual pages must be cleared as the problem occurs.