Like plugin shows an advertisement instead of the Like button

I have the Like plugin installed on one of my sites, where it works fine. But on another site though, when enabled it shows the top of an advertisement instead of the Like button. Another similar plugin, Facebook Like, does the same thing.

Both sites are hosted on the same provider, HostGator. Anyone have any clue as to what might be causing this weird behaviour?

Also, if I enable Use XFBML, the button disappears altogether. I’ve never been able to use XFBML with any Facebook Like plugin; I’ve created an app and put the app ID in. Has anyone else managed to get XFBML to work?

Thanks,
Graham