That's according to adherents.com, which "estimates" the worldwide religious population.
Now some of those "estimates" should probably be taken with a grain of salt as big as the Biblical Mrs. Lot.
For example, the Catholic Church has been claiming 60 million American members for DECADES now...ever since the 1940's, I think. You see the "60 million" number thrown around by Catholic DU'ers all the time.
But that represents the number of BAPTIZED Catholics, not currently practicing members. And Catholics are baptized as helpless infants, when they can't protest. AFAIK, the Church doesn't track how many people leave. But we can guess from the news stories about dioceses closing down and churches going empty in Boston--one of the most Catholic parts of the country.
The Top Ten:
1. Christianity: 2 billion
2. Islam: 1.3 billion
3. Hinduism: 900 million
4. Secular/Nonreligious/Agnostic/Atheist: 850 million
5. Buddhism: 360 million
6. Chinese traditional religion: 225 million
7. Primal-indigenous: 150 million
8. African Traditional & Diasporic: 95 million
9. Sikhism: 23 million
10. Juche: 19 million
You can see the whole list here:
http://adherents.com/Religions_By_Adherents.html