My kids are still youngish--14 and 11--and they both have facebook accounts, and the accounts, like their email accounts, were established with the knowledge that both parents would have access to the accounts.
The youngest only really uses facebook to keep in touch with the grandparents. She decided she doesn't like having email, so she just uses my email account when she needs to.
I tell that that I check their accounts from time to time, but mainly it's just their walls. I also tell them how much I appreciate that they appear to conduct themselves with intelligence and dignity on facebook.
FWIW, they also have access to our facebook accounts. They know both my husband's and my email passwords, our facebook passwords, and the oldest knows our bank account password. Some of this has to do with some individual international travel that occurred, and we wanted the oldest to have access to money and information should she need it. They are trustworthy kids, and honestly, I wouldn't feel violated if they checked my email or facebook. I don't have anything to hide, and I've told them this.
With the oldest, we recently discussed if she was still okay with us knowing her passwords. She said she was fine with it. I don't think we abuse her trust, and since she knows our passwords and doesn't abuse this, I think we're kosher here.