No, building data centers is not your core business, but your core business relies on them so intimately that it makes sense (at least to some people) to have a little more direct control of your datacenter resources. Or, the have a warm-standby of some sort.
The best analogy I can make is it's like a telemarketing company. Their core business is not building/owning/maintaining PBXs, it's calling you and trying to sell you crap at diner time. However, because they cannot deliver their core product reliably without a PBX, they tend to own/maintain their phone switches. They don't order 300 simple phone lines from the local telco, they order a couple of trunks, and manage phones and call routing to fulfill their core business needs.
It's not a perfect analogy, I know, there are cost factors unaddressed, but it's the best I can come up with right now.
I'm personally a fan of cloud services for elasticity, and early stage launching. I'm not a big fan for day to day operation of a site. The model I haven't seen worked up yet, it what does this REALLY cost a site like reedit or 4sq in relation to owning/controlling more of their own infrastructure? It's quite possible the best overall business decision is to take the risk and deal with the unpredictable inevitable outages and just hope they don't last too long.