If it doubles each year, it means that the next year the number will be 2 times its size.
So if in 2005, the number of users is 2 million, then in 2006 the number of users will be 2 times the number in 2005.
Knowing this, we then know that the year 2004 will be half the users of 2005. Then the year 2003 will be half the users of 2004 and so forth.
So you solve for each year as such:
2005: 2
2006: 2 x 2 = 4
2007: 4 x 2 = 8
2008: 8 x 2 = 16
2009: 16 x 2 = 32
2004: 2 / 2 = 1
2003: 1 / 2 = 0.5
2002: 0.5 / 2 = 0.25
2001: 0.25 / 2 = 0.125
