As of September 2007, approximately 1,244,449,601 people worldwide use the Internet, according to Internet World Stats.
http://www.internetworldstats.com/stats.htm
If you wanted to do a very crude approximation, you could divide that by the %s of each, but it would be mindbogglingly crude.
People frequently claim a religious affiliation when what they really mean is a cultural one.
Overwhemlingly they will be in industrialised nations, and therefore there is going to be some degree bias towards Christianity, Judaism, Confuscianism and atheism.
Plus, it's not a requirement to state religious affiliation to use the net...
This is how it works out though:
Christianity 620.1 m
Islam 300.4 m
Hinduism 213.3 m
Buddhism 88.5 m
Sikhism 5.1 m
Judaism 3.8 m
Bah�'� Faith 1.7 m
Confucianism 1.5 m
Jainism 1.4 m
Shinto 0.7 m
But I wouldn't put much faith in those figures myself. They're accurate as a reflection of total Internet users split by major world faiths, but hopelessly inadequate in that they don't take account of the other variables.