Here's something I found on Wikipedia about the popularity of religions around the world:

Christianity 2.1 billion (see below)
Islam 1.3 billion (see below)
Non-Adherent (Secular/Atheist/Irreligious/Agnostic/Nontheist) 1.1 billion
Hinduism 900 million (see below)
Chinese folk religion 394 million (see below)
Buddhism 376 million
Primal indigenous ("Pagan") 300 million
African traditional and diasporic 100 million
Sikhism 23 million
Juche 19 million
Spiritism 15 million
Judaism 14 million
Bahá'Ã* Faith 7 million
Jehovah's Witnesses 6.5 million
Jainism 4.2 million
Shinto 4 million (see below)
Cao Dai 4 million
Zoroastrianism 2.6 million
Tenrikyo 2 million
Neo-Paganism 1 million
Unitarian Universalism 800,000
Rastafari movement 600,000