Found this great "contract" you should consider signing with your kids...read it carefully because it will help you know what you should be monitoring and to what extent.
My manager the other day was telling a story about an e-mail his 13 yo daughter had received. My coworkers were appalled that he and his wife read her e-mail...coworkers who did not have children and are not aware of the sometimes heavy responsibility it is to raise children in this digital age.
My bottom line to you...be present in your children's lives, know what they are doing online!
Kim Komando's 10 Commandments for Kids Online
#10: I give my parents’ permission to look on the computer and my phone to see where I have gone on the Internet, the e-mail and text messages I have sent and received, the things I downloaded or what I do. If my parents installed programs that track what I do on the computer or limit where I go online or on my cell phone, I promise not to turn those programs off.