Without qualification, Android has been a viral success story. It reached one billion activations in about five years, almost half the time it took FaceBook and far faster than Windows
Android is the third platform to reach a billion users[1] . The first was Windows and the second was Facebook. Apple sold around 650 to 700 million iOS and is expected to be the fourth to a billion sometime next year.[2]
If we define the Race To a Billion to be bounded by a time limit of 10 years, then Windows does not qualify and Android is actually second. The race is shown in the following graphs (the one on the left is logarithmic scaled, the one to the right includes only a few contenders for illustrative reasons).
This Too Shall Pass 