Short answer: Very carefully.

Long answer: They have networks of carefully crafted databases on many many servers. No one server could ever hope to serve that much traffic. They have exprets doing this, facebook as it is could never be created by anyone but advanced programmers. They, like all not small systems, would use realational databases. But that is just the start of it, how you relate the keys is important, more complication comes up when you have hundreds of servers trying to act as one database on the outside.

I've personally never worked with databases took up more than a single server, so I don't know too much about how they do it. Books have been written on it, I would seggust searching amazon.com for some.

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