Depending upon whose estimates you believe, Monday may or may not have marked the birth of the 7 billionth human living on earth. The US Census Bureau estimates that the 7 billionth resident of earth will not be born until March of 2012. Either way, this is a remarkable population expansion in a short period of time.
The world’s population has grown exponentially in the last 100 years, but the rate of growth has declined drastically in recent decades and is expected to continue decreasing through the middle of the 21st century. The world’s population first surpassed 1 billion people in the early 1800’s and 2 billion people around 1930. In less than 30 years, the world population topped 3 billion (1959). It took less than 15 years to exceed 4 billion people (1974), 13 years to reach 5 billion (1987), 12 years to reach 6 billion (1999) and 12 to 13 years to top 7 billion (2011 or 2012). However, current projections estimate that it will take at least 14 years to reach 8 billion people (somewhere around 2025 or 2026) and 16 more years to reach 9 billion people (in 2041 or 2042).
The charts below are based on US Census Bureau and UN population estimates.

Chart showing growth in world population

World population growth spiked after 1900 and has just started to flatten out.

World population and growth rate 1900 to 2050

Rapid growth in world population is expected to subside in the next 40 years