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Is it because more black celebrities like Sean "P Diddy" Combs are using Twitter?
"Perhaps because Twitter is most popular amongst 25-34 year-olds. The median age for black Americans (according to the 2000 census) is 30 years old, a full seven years younger than for white Americans. Black people therefore make up a relatively higher percentage of the population within the most relevant age groups."
Or perhaps...
Business Insider has a few more ideas on why Twitter is more popular with people of color.