This is happening because there is another Facebook javascript library being included on your page, all.js. The all.js extension (probably included by your Fanbox module) is the 'old' version of the Facebook Javascript API which was discontinued a long time ago. It's no longer meant to be used and can cause conflicts with some JFBConnect features, as JFBConnect uses the recommended sdk.js library. These two libraries do not work together nicely. If you disable the inclusion of the all.js javascript file, the Facebook functionality should work again, since JFBConnect will be adding the recommended library.
Here's the code on your page showing the inclusion of the library:
<div id="fb-root"></div>
<script>(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/fr_FR/all.js#xfbml=1";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>
-Melissa
Use another facebook extension
