Ask HN: How can Facebook go down?

Surely Facebook's server code and databases are replicated around the world, partly for redundancy and partly for faster content delivery. So presumably a datacenter to burn down (or a big storm can hit the US) and Facebook could keep ticking over. What, then, could cause the whole thing to go down? I'd love to hear people's thoughts.

Perhaps the fault was with Akamai, but then my question really just changes to 'How can Akamai go down?'