Well lets see. One joint doubles your chance of schizophrenia, 1/3 of Americans have admitted to using cannabis at some point in their lives, and the number is probably larger. That means 100 million people should be at double risk for szhizophrenia. Checking out Schizophrenia.com - Schizophrenia Fact and Schizophrenia Statistics , the following is their explanation, copy/pasted for the cause of schizophrenia.
Today the leading theory of why people get schizophrenia is that it is a result of a genetic predisposition combined with an environmental exposures and / or stresses during pregnancy or childhood that contribute to, or trigger, the disorder.

But more importantly, relevant to the original topic, I took the following from their site:

The Prevalance Rate for schizophrenia is approximately 1.1% of the population over the age of 18 (source: NIMH) or, in other words, at any one time as many as 51 million people worldwide suffer from schizophrenia, including;

6 to 12 million people in China (a rough estimate based on the population)
4.3 to 8.7 million people in India (a rough estimate based on the population)
2.2 million people in USA
285,000 people in Australia
Over 280,000 people in Canada
Over 250,000 diagnosed cases in Britain


So at 2.2 million out of 100 million Americans who have used cannabis, and assuming (to be generous to our prohibitionist friends) every single person with schizophrenia has used cannabis, it would seem the prevelance of schizophrenia among (minimum one-time) cannabis users is approximately 1/50.

Facts don't quite add up do they?