Time, Nov. 28, 1955: According to estimates by the American Musical Conference, the current rankings for numbers of US players by instrument (or type) was as follows: 1st: piano - 19,000,000, 2nd: guitar - 4,000,000, 3rd (tie): all woodwinds and brass - 2,000,000 each, 4th: ukulele - 1,600,000. (link)
See also a related blurb on uke sales: link
Time, March 12, 1928: Honolulu instrument maker George P. Mossman (aka "the Hawaiian Strad") is reported to have created a ukulele that can be heard 1/2 mile away.