I am from San Francisco, so I got curious about your question and decided to do some research. It wasn’t easy finding statistics on this level.. the government doesn’t differentiate Chinese languages on the census. This article is a little dated (2003) but it can give you a general idea, I think.
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Chinese communities shifting to Mandarin
A gradual shift from Cantonese, a dialect spoken in southern China, to China’s official language of Mandarin, has been taking place in America’s Chinese communities. These days, Mandarin’s growing influence can be heard even in San Francisco’s Chinatown, long a bastion of Cantonese speakers.
“Now, nobody pays attention because it’s so common,” said Pak, a longtime Chinatown activist and consultant for the Chinese Chamber of Commerce who speaks both languages. Though Cantonese remains Chinatown’s primary tongue, many shopkeepers speak at least a few words of Mandarin.
Statistics document the shifting landscape: A 1986 consumer survey found almost 70 percent of Chinese households in the San Francisco area spoke Cantonese; 19 percent spoke Mandarin. A survey last year showed the divide narrowing to 53 percent Cantonese and 47 percent Mandarin, according to a study for KTSF, a television station that devotes most of its programming to Asian-language shows.