Online behavior is not the cause. It might be an aggravating factor, but teenagers have been sleep deprived for a lot longer than the internet has been around [1]. If we want to talk personal anecdotes, I
definitely never slept enough as a teenager despite not having internet access at that point. I'd be up reading books and stuff like that.
There are a lot of factors that go into adolescents not getting enough sleep, and it's pretty clear that early school start times are one of them: and one that can be controlled.
Also I think suggesting that parents can take responsibility for the bed time of their teenagers is more than a little unrealistic.
[1] http://www.sleepforscience.org/stuff/contentmgr/files/94a9f6... (from 1990 about sleep deprivation in adolescents)