"Unlike Google and Facebook, AWS doesn’t aggressively brand or call attention to their data centers. They absolutely don’t give tours, and their website offers only rough approximations of the locations of their data centers, which are divided into “regions.” Within a region lies at minimum two “availability zones” and within the availability zones there are a handful of data centers."

I want to point out one very important fact. AZ is logical. A typical AWS account would get, IRRC, up to 4 AZs. However, not all accounts is provisioned with the same set of AZs.

Account A: a, b, c, e Account B: a, c, d, e Account C: a, c, d, e

By logical, even though all three accounts have AZ "a", that "a" is not necessarily from the same data center (remember multiple DCs per region). It can be "a" in one of their us-east-1 DC#2, or DC#3. Though when m3.medium runs out in us-east-1 AZ "a", you will get full unavailability in all accounts running "a".