On Internet, websites keep on materializing and vanishing every day. Some of them reappear in a new avatar with new looks and sometimes even with a new name! Changing domain name of a well established website is never advisable –but it has to be done sometimes. Even if you don't change domain name –other changes in your website's URL structure are quite common as well as sometimes of necessary.
If you have placed a Like button on your webpage and have received a lot of likes –any change in the URL of that page will wipe off your Like count. Facebook does not track changes in URLs. It simply keeps the count associated with a particular URL –which means if URL is gone (i.e. changed) –the count shall also be gone.
And there is not much we can do about it. So, my first advise would be that you try not to change URL. But if you have to change it, there is a workaround to get your Like count back. It may or may not work in all cases. It has worked for me recently –so I just thought of sharing information with you!
After changing the URL and placing a brand new Facebook Like button on your webpage –you must set the old URL to a 301 redirect so that old URL begins to redirect to the new URL.
Now ask someone who had liked the page before to like it again. That’s it! Facebook will recount the likes associated and you will get your Like count back.
I am not sure if it works in all cases. As I said, it worked for me but it is still just a work-around. I hope it works for you. If not, as I said, there is nothing we can do about it. Facebook should make necessary changes in their Like code to accommodate fluid nature of the Internet.