If you follow the link you are assigning to the iframe src you will see nothing but a blank screen. Therefore you see nothing on your html page.
You can generate Facebook share button by using this:
https://developers.facebook.com/docs/plugins/share-button/[
^]
Here is an example how to embed a generated share button:
<html>
<body>
<iframe src="https://www.facebook.com/plugins/share_button.php?href=https%3A%2F%2Fdevelopers.facebook.com%2Fdocs%2Fplugins%2F&layout=button_count&size=small&width=96&height=20&appId" width="96" height="20" style="border:none;overflow:hidden" scrolling="no" frameborder="0" allowfullscreen="true" allow="autoplay; clipboard-write; encrypted-media; picture-in-picture; web-share"></iframe>
</body>
</html>