I'm assuming that by "making public statements", you mean "showing off." (Not that that's a bad thing; it's also known as "signaling." [1]) The people teens are concerned about showing off to are always in physical proximity to them; they don't need an out-of-band channel for this. Adults are much more physically disconnected from the people they seek to impress. This encourages things like Twitter.

[1] http://www.overcomingbias.com/category/signaling