How are you adding the tag? Are you using the {JFBCLogin} tag, or using your own custom image? The {JFBCLogin} tag is the simplest way to add the button, but uses Facebook's Javascript button, which comes with a bunch of CSS and take a second to render. The image is more flexible, but takes more code. To use a custom image, use the following HTML:
<a href="javascript:void(0)" onclick="jfbc.login.login_custom();"><img src="/linktofbimage.jpg" /></a>
If you need help diagnosing the CSS, we'd need to actually see the page. Can you let me know a page where we can see the issue on? You can post it here or Private Message it to me by clicking the mail icon under my picture to the left.
Thanks,
Alex