Although there are no statistics available for the years prior to 1901, baseball historians and statisticians agree that attendance at baseball games boomed during the 1900s. In 1901 attendance was 3.6 million, an average of 225,000 for the sixteen major league teams. By 1910, the first decade of play by both the American and National Leagues, attendance had doubled to 7.2 million. The primary causes for this increase were the effective control of vulgarity and unseemly behavior on the field, a series of outstanding pennant races, and huge popular interest in the World Series. During the 1900s the New York Giants drew the greatest number of fans, 4,977,481, with 910,000 in 1908.

Source:

Hy Turkin and S. C. Thompson, The Official Encyclopedia of Baseball, eighth revised edition (South Brunswick & New York: A. S. Barnes / London: Thomas Yoseloff, 1976).