How to add facebook like button after read more

I'd like to add a facebook like button at the end of my excerpt, right after the read more text.
I would like to have it there, so it doesn't go below the post image, which is what happens if I put it in the footer.
I'm using the iframe code, since I want to limit the number of plugins. I tried to paste the iframe code at the end of the custom read more code, but I get a strange behavior.
Where should I put this code?