ABC affiliate WJZ-TV (now WABC-TV) in New York City went on the air August 10th, 1948. According to C.E. Hooper, from 7-10:30PM that night the station drew a 29.3 rating and a 52.3 share of the audience. During that period, 56.5% of television sets in the area–some 200,000–were in use.
Doing a little math, that means about 113,000 sets were in use from 7-10:30PM on August 10th, 1948 and roughly half were watching WJZ-TV. Not bad for a new station.
“Radio and Television.” New York Times. 18 Aug. 1948: 50.