Unfortunately, you can't. A Facebook application can only be associated with one domain, so that when a user 'accepts' your application, it's for that domain. It will work across subdomains and directories, but not multiple sites.
If you weren't looking for authentication, the social plugins should all work on multiple sites without issue, but usually, authentication is a big part of what user's are looking for.
Hope that helps, but if you need more specifics for a certain usage case, just let us know,
Alex
Error in two domains (.com and .com.br)
