tangor - 9:46 pm on Jan 18, 2014 (gmt 0)
Does that mean it's losing existing users or only failing to acquire new ones in the specified age brackets?
A difference. The teen of 2011 is not a teen in 2014... but during that time any new signups in the teen category...
FB is LOSING new teen signups. We don't know if the existing teens have fled (though I suspect they have). What FB is losing is the teen market where they hoped their targeted ads would have value. Hence, the drop in FB valuation on the market.