News of Apple’s plans for a $1 billion data center to bolster its web applications coincides with Om’s GigaOM post about the exponential growth of Twitter and the infrastructure investments that accompany that growth. Both of these situations help explain why the won’t likely consist of a few large public clouds used by everybody, as some have proposed. Popular web sites have too much traffic to run on anything but a custom-built infrastructure, and the number of insanely popular sites keeps growing. Would you trust a cloud provider to meet its SLA for your app if that same infrastructure also had to handle Twitter’s 32.1 million-and-growing unique visitors per month, and maybe the load of another similarly popular site? Or vice versa?