Answer: 1,000,000,000 unique pageviews in a single month. According to PCMag.com, "Traffic to Google properties has increased by 8.4 percent over the past year to reach the billion hit mark. Microsoft has maintained the second-place spot with 905 million unique visitors last month, a 15 percent increase. Facebook takes the No. 3 spot with about 714 million visitors, up 30 percent. Yahoo comes in at number four with 689 million unique visitors in May."
For people (like me) whose job it is to drive traffic, the achievement is amazing...and depressing.