According to new estimates by eMarketer, Facebook users in the 12- to 17-year-old demographic declined by 9.9 percent in 2017 , or about 1.4 million total users. That's almost three times more than the digital measurement firm expected. There were roughly 12.1 million U.S. Facebook users in the 12- to 17-year-old demographic by the end of the year. Recode reports: