Multiple domains - One site

Yes, in that case, you'd need 5 different apps. Facebook only allows an app to work on one domain (but can work across all of it's subdomains). There's no way to make 1 app work across multiple root domains.

Unfortunately, JFBConnect only works with one App ID/key as normally Joomla only powers one domain at a time.

With that said, if possible, I'd really rethink the multiple domains thing for a few reasons:
* If your content is spread across multiple domains, you're loosing a whole ton of search engine power since links to your site won't be focused and will be spread across them all
* With multiple FB Apps, a user will have to approve each app the first time they go to a 'new' site and hit Login With Facebook

Finally, with all the above said, the standard Login with Facebook functionality does not require an SSL certificate at all. The only features that require SSL are Page Tab or Canvas integration (showing your site within Facebook) or the Facebook Requests (invitation) features.

Hope that helps!
Alex

It sounds like it should work. Obviously, yours is a somewhat different scenario than normal, so you'll need to test a lot. I'm not exactly sure how you'll have site2.com redirect the user login to site2.parentcompany.com and stuff, but I'm assuming you have a bit worked out behind the scenes.

Certainly, if you run into issues, just let us know!

Good luck,
Alex