We expect websites to be available when we visit them. And they usually are. Once is a while though the site you're trying to visit won't load or is so slow you get tired of waiting and move on. We proactively monitor your website to mitigate attacks you may not be aware of.
There's lots of reasons a website won't load - equipment or network failure are ones you're probably familiar with. One reason you might not heard of is a Denial of Service (DoS) or Distributed Denial of Service (DDoS) attack. The following video describes DDoS attacks against WikiLeaks.
These attacks are far more common than you might expect. From time to time we hear about attacks against major websites - banks, Amazon, PayPal and the like. Sometimes these attacks are, at least for a time, reasonably successful - websites are so slow to load that visitors just assume the site is down. And these major players invest serious resources in protecting themselves.
There have been attacks against our servers and we've suffered during the worst of them. But a combination of hardware, software and multiple networks has enabled us to minimize the impact and get things back to normal pretty quickly.