They have their own data centers, they don't use AWS

I'm less surprised if AWS internally pays AWS list prices, because that's just internal accounting. From the even relatively small AWS customers I know, none of them needed to get very far into the 6 digits per year spend before a couple of quiet mentions to their account manager that they were reviewing other options was enough to get steep discounts.

Add in lots of credits, and if you pay list price, you're being taken to the cleaners..

I've done contract work for clients to be ready to migrate both as part of maximising credits and as part of negotiating posture, and the savings can be enormous (though it'd still usually be cheaper to use managed servers).

Couple of reasons: Meta's annual spend with AWS is large enough that they'll have a negotiated blanket discount that takes a fixed percentage off the top of their monthly spend. This is very common for larger AWS customers, not just Meta.

For instances specifically, any planned usage will be using either reserved instances or at minimum a compute savings plan (CSP) that drops the hourly rate dramatically in exchange for a committed number of instance hours, with or without an upfront payment.

Finally, there may be a negotiated rate for specific instance types built into the contract. Again, common for very large customers.

source: I was on one of the cloud-related infrastructure teams (left in early 2022). I have no idea about their spend (or discounts) today, but two years ago it was enough that Andy Jassy would meet 1:1 with Mark to "discuss the relationship".