My thought was that they want enough separate cores to handle a case where 1 core is handling 1 user's page load.
So for a midsized datacenter with say 200 racks, each ARM based rack could have 320 cores in it, giving 64K cores.
Since a FB page may take 200ms to load (or some other load time internal to FB response time targets), such a setup could handle 64K * 5 (i.e. 5 users per second at 200ms/page) or 320K simultaneous users per second; figuring 30 seconds of viewing time per user, this gives you almost 10 million users' worth of capacity.
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My thought was that they want enough separate cores to handle a case where 1 core is handling 1 user's page load.
So for a midsized datacenter with say 200 racks, each ARM based rack could have 320 cores in it, giving 64K cores.
Since a FB page may take 200ms to load (or some other load time internal to FB response time targets), such a setup could handle 64K * 5 (i.e. 5 users per second at 200ms/page) or 320K simultaneous users per second; figuring 30 seconds of viewing time per user, this gives you almost 10 million users' worth of capacity.
(all numbers back of the envelope, hypothetical)