The Nielsen universe is shrinking. Just in time for the upfronts, where TV networks sell advertisers means to reach potential TV viewers, Nielsen announced that the number of those potential TV viewers has dropped for the first time in two decades. The latest data released today shows that 96.7% of U.S. homes own a TV set, down from 98.9% last year. There are now 114.7 million TV homes, compared to 115.9 million in 2010. Nielsen notes that the last dip in the number of TV homes came in 1992, when the country was also in a recession, and it was temporary. The current economic woes are one hypothesis for the decline as low-income homes — particularly in rural markets — can no longer afford TV sets. As cynical as it may sound, advertisers likely wouldn’t care much about that. But another possible reason for the drop — that college graduates or other young people entering the workforce don’t buy a TV set as they may be watching TV on their computer or shun TV altogether for other forms of digital entertainment — will probably deeply concern them.