Nielsen has started tracking shows on Amazon, Hulu, and Netflix

When asked recently if she knew how many people were watching the comedy “Unbreakable Kimmy Schmidt” on Netflix, co-creator Tina Fey said she had no idea. “We don’t have any actual numbers,” Ms. Fey said at a recent gathering of television critics. “I feel a lot of people are watching the show. Let’s go with that,” she cracked. Netflix Inc. is notoriously secretive when it comes to sharing such information—be it with content providers, the media or Wall Street—arguing that since it doesn’t have advertisers to please, ratings for its subscription streaming service are irrelevant. But now Hollywood is getting its first real peek inside the black box of online video streaming. In recent months, measurement specialist Nielsen has been scaling up a program to track viewing on Netflix and other online services like Amazon.com Inc.’s Prime Instant Video and Hulu. Nielsen said it is now tracking nearly 1,000 shows. Most major TV studios are receiving detailed readouts on how their own programming is performing, including the total viewers for any episode and basic demographics such as age and gender, Nielsen said.