More that one study, more that one observer, more than one predictor has noticed that teens as early as 2005 were abandoning email. Chad Lorenz writing in Slate spent some serious time in anecdotage whilst writing his article, The Death of E-Mail
Teenagers are abandoning their Yahoo! and Hotmail accounts. Do the rest of us have to? back in 2007, but he also cites some of the early Pew studies on teens and email.
Lorenz would be even more pessimistic about email if he could have leapt to the future and read this 2010 report on teens and their communication choices.