NOP 3.0 - Multi-Store & Facebook authentication return URL:
When I think of multi-store, I think of the ability to use multiple TLD's and/or sub-domains to direct customers to the store of their choice. That being said, there should be an option for store-specific Facebook authentication app keys & secrets so that the return url configured at Facebook for each auth app will return the customer to the specific TLD or subdomain they were referred from.
On the administration back-end, you wouldn't have to create another customer account for each store, just allow multiple Facebook Authentication Identifiers for each store they've registered with.
It's possible that I'm over-thinking this and if it isn't an issue for anyone in their multi-store test environments, please let me know!