D.P.
Melanie,
How old are your kids? From your profile, it looks like O. is 6 (?). I assume these are older kids you're asking about?
Ok--so you're asking about teenagers. Sometimes it's not just as simple as "laying down the law" as suggested. PERIOD! LOL
There are software programs for blocking of questionable content...Besecure.com, etc.
You can always check your computer's history.
You can have the kids leave the ipods, phones, etc in a common living area when they go to bed at night.
You can also have them change their FB accounts to a new (mom controlled) password so the only option they have is to use FB in public home areas.
You can talk to your cell providers about restricting texts, phone numbers, etc--I have a coworker that did this recently.
It's crazy out-of-hand, the teens and 20 somethings GLUED to their devices, phones, etc....BUT technology IS NOW and these kids need to "be up" on it and embrace technology to the fullest. It is a HUGE part of the work world, so in a way, it's a good thing for them to be "knee deep" it just has to be monitored and safe, as you said.
ETA: Here's a previous thread that you might find useful:
http://www.mamapedia.com/questions/12793391768569708545
Apparently--there's an app for that! LOL