If the sites are all on different domains, you'll need to have different applications for each. If they are on the same domain, but each site is a domain, you can use the same application. That's a requirement of Facebook.. an app only works with one domain.
As for posts, comments, and likes, those are almost all unique to each URL and not dependent on the application you create with Facebook. The app is more important for the Login functionality and other things.
As for having different Fan Pages, you can use the JFBCFan module to display info about that Page on all 5 sites without any problems. Additionally, you'll be able to create "Page Tabs" (those little boxes under the cover photo on your FB Page) for each of your websites. That will let you show content from each of your sites in your Facebook Page itself.
Hope that explains!
Alex
Facebook Page, Apps and Sites
