Hello,
Sorry it is not possible to achieve that. Google plus uses different method to show the popup and it is that button which automatically detects the corners. I didn’t do any manipulations there.
But, in FB like the popup is displayed within a iframe. It is not possible to manipulate a DOM element within an iframe from a parent window.
So either FB should design the buttons as Google+ buttons do. It is not possible to manipulations directly on the popup window.
Anyway the like will be registered on Facebook.
Thank you.