The button is in 1 line now, but if you load the site, it pushes other divs down, then "recovers" to one line. As far as I could see FB is not picking up a height from one of the divs. I found this instead:
"The plugin dynamically sizes its height; for example, if you specify a maximum of four rows of faces, and there are only enough friends to fill two rows, the height of the plugin will be only what is needed for two rows of faces." http://developers.facebook.com/docs/reference/plugins/login/
Trying to tell Facebook not to show any friends: <div class="fb-login-button" data-show-faces="false" data-width="100"></div>
does not change anything. Still have this unnice effect (this is one reason why I removed the FB-login now).