From the NAS bimonthly newsletter "Population & Habitat Campaign July/August 1999" (Data is for the month of June):

World Population: 5,993,166,860	U.S. Population : 272,753,842
One birth every: 8 sec.	One death every: 15 sec.
One international migrant every (net): 30 sec.	One U.S. citizen returning every (net): 4781 sec.
Net gain of one person every: 11 sec.