How TikTok grew from a fun app for teens into a potential national security threat

SAN FRANCISCO — If it feels like TikTok has been around forever, that’s probably because it has, at least if you’re measuring via internet time. What’s now in question is whether it will be around much longer and, if so, in what form?Starting in 2017, when the Chinese social video app merged with its competitor Musical.ly, TikTok has grown from a niche teen app into a global trendsetter. While, of course, also emerging as a potential national security threat, according to U.S. officials.On…