The world population was estimated to have hit 6 billion people on October 12th, 1999. It took thousands of years for us to hit 1 billion people according to Earthsky.com, which was estimated to have happened around 1804.
At that point, the population continued to grow exponentially, which took the world population to 3 billion by the year 1960. Doubling that only took about 40 years, which marked the 1999 milestone of 6 billion. In 2011, we jumped up to the 7 billion mark worldwide. We are estimated to be over 7.3 billion as of today.
Although these numbers are all estimates, the explosive population growth has not slowed. Current estimates from the United Nations report, World Population Prospects, puts the world population around 8.5 billion people by the year 2030, with an estimate of around 9.7 billion by 2050. By 2100, the world population is forecasted to be around 11.2 billion.
Right now the United States is the 3rd most populated country, with an estimated population of over 325 million people. China is the most populated, with and estimated population of 1.4 billion and counting. India is expected to pass China as the country with the largest population around the year 2022.