A report on Friday by Frank N. Magid Associates Inc. found that the portion of 13-17-year-old social-media users in the US on Facebook slipped to 88% this year from 94% in 2013 and 95% in 2012. In the same period, Twitter Inc. and messaging applications rose in popularity in that age group, the study showed. The Menlo Park, California-based company first warned a year ago that teens weren’t using its website as often as before. Facebook stopped discussing teen usage on its earnings calls after last year’s disclosure alarmed investors. While the issue was all but forgotten as the company’s advertising revenue reached new highs, it’s a bigger concern now, according to Tero Kuittinen, a managing director at Magid in New York.