It’s estimated that there were 1 billion human beings in 1800. We made it to 7 billion in 2011. With that many people doing what people do, we’ll hit 8 billion in another 15 years or so, assuming nothing out of the ordinary happens.
The Atlantic has “A Real-Time Map of Births and Deaths”. It’s an impressive visual simulation of the world’s rapid and uneven population growth. It’s based on statistics, of course, since we haven’t yet reached the point where every birth and death is instantly recorded (unless someone isn’t telling us).
By the way, the United States is the 3rd most populous country in the world, after China and India. For some reason, I find that surprising (maybe I got used to the Soviet Union being in the 3rd position). The Census Bureau provides the latest estimated numbers for America and the world on their population clock: http://www.census.gov/popclock/