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In bad news for Facebook Inc. (NASDAQ: FB), the percentage of teens active on the social media platform fell by 9% last year, according to a global survey of more than 40,000 Internet users ages 16 to 19 by research firm GlobalWebIndex (GWI). Twitter Inc. (NYSE: TWTR) does not have it quite so bad, as the number of teen users fell by only 3% in 2013.

However, the numbers of teens with accounts on these sites remained largely stable, suggesting that they are not abandoning the big social platforms, just using them less frequently.

So where have they gone? To mobile messaging apps. Here Facebook is on better ground, as its messenger app remains the most popular with a 25% share. WhatsApp and WeChat are each used by 20% of teens globally. Note that Snapchat saw growth of 60% in 2013, the fastest rising messaging app, according to GWI.

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The study also found that teens lead the total online population in use of the Internet on a mobile device, with 68% of them mobile Internet users. That was up 5% last year. And almost all teens are second-screeners; for example, chatting on a their phones while watching television or playing video games.

GWI found that Google Inc.’s (NASDAQ: GOOGL) Android is the operating system preferred by the teen demographic, at 62%, compared to just 19% for Apple Inc.’s (NASDAQ: AAPL) iOS. Android use among them grew by 17% between 2012 and 2013.