A bit of a late reply here, but I think we are often guilty of allowing kids too much internet time. Our 17 year old spends the most time online, usually in the evenings. There's an awful lot of Facebook activity involved.
Being her official "IT Manager" I've never seen anything that has caused me any concern in her online activity, but the sheer number of hours spent online is beginning to be a topic of conversation around the dinner table.
16 y.o son also seems to go online at ridiculous times of day (even found a message posted on his Facebook Page at 5am the other day!!!), but his recent discovery of cycling has ensured he spends much less time online than he ever used to.
At their age, I think just highlighting your awareness is probably more effective than enforcement. They're very canny these teenagers, so a simple chat saying that you're monitoring their activity is much less likely to trigger a strop than denying them opportunities. At least they get the chance to think things over in their own time.
As far as what they do online is concerned, just have a regular check of their internet browsing history to make sure they're not going places they shouldn't - and if there is no history, ABSOLUTELY HIT THE ROOF WITH THEM
and threaten to withdraw their privilege completely (for obvious reasons!). If there's no history, they're difinitely hiding something!
There are many parental monitoring software packages available that will keep an independent record of browsing history that won't be washed away when a browser's history is cleared. Worth considering if anything suspicious comes to light!