IPv6 – Why There Are No Different Classes of Network

MAC addresses only need to be unique in a local broadcast domain, not globally, so re-use of MAC addresses in different networks usually isn't a problem.

The internet isn't one global broadcast domain and thus needs to be divided into many blocks of addresses assigned to different ISP's and each ISP divides his blocks into smaller blocks for different customers/services. To allow each of these smaller blocks to contain many MAC addresses you need to have the IP-space much bigger than the MAC address space.

Unfortunately, Cisco did away with wildcard masks in IPv6. That is mostly a good thing, EXCEPT in this particular case. For your idea to work however, you have to rely on Facebook being both "clever" and consistent, which is probably more than one can hope for.

But if you want to process Facebook's traffic differently than other traffic, you can simply filter on their assigned address block. The one you mention in your question is actually assigned to Facebook Ireland: 2a03:2880::/32.

But it is just as easy to look up others in the registries.