When you think about a site like YouTube, you can easily lose sight of just how huge its presence is on the internet. Google is announcing that YouTube’s monthly unique visitor count just cracked one billion for the first time — nearly one out of every two people on the internet. Even more interesting is how quickly YouTube keeps expanding its reach. A year ago, it was just scratching 800 million unique visitors — that’s 25 percent annual growth for an eight-year-old site.