Of the few hundred friends I have on Facebook (almost all under 18), not one is using a pseudonym. Not sure where you heard that.
Here's an old article mentionning this fake name trend: http://www.guardian.co.uk/society/2011/oct/02/facebook-child...
I am not a teen anymore but I find this surprising because IRC, MSN was all the rage when I was younger and nickname was really the norm. Just like those _xx_daemon_xx_@hotmail.com addresses.
I am a supporter of half-anonymous practices and I am surprised it's not more widespread than I believed.
Update: And from http://www.pewinternet.org/Reports/2013/Teens-Social-Media-A... I can see fake names seem to be marginal, with people managing at least one real/official account.