It is a problem, but if caught early enough then kids understand.
my 13yo has had fb for just under 2 years, her mates had it when they were in P6, which put a lot of pressure on us, but we discussed and agreed a compromise of when she's settled into high school.
Her account is tied to her email, which she doesn't use, but is setup on my laptop. We know her password, if she tries to change it, I get notified and she gets a short ban. It also means that whatever is posted up by her close friends gets emailed to her account, so we can monitor that if need be.
There have been issues of bullying by others on there, we've stepped back to see how she deals with it, if she shows signs of distress we intervene, removing 'friends' and in some cases blocking them.
most of fb is either boys posting suggestive quotes and girls posting about how crap their lives are because x did x and mums not letting me pierce the new fashionable bit of body that's probably inappropriate for a 12 year old to pierce....
Certain undesirable influences went on within the peer group which got police etc involved so we keep a fairly close eye on things now. FB isn't the problem, BBM is. We can monitor her BBM/texts and gps locate her now if we need to, but thankfully there have only been a handful of times we've had our suspicions raised so much we've needed to. We're pretty glad we checked but it's not really the finding out if there are social issues fueled by social networking site/apps, it's more about what you do with the information when you have it.
the sites and apps are going to be a part of it, make it forbidden and kids will find ways to get around it. Anything that we find out we discuss with our daughter, verbally roleplaying scenario's we see, but she is unable to at the time. It's not about telling them what to/not to do, it's about guiding them to your way of thinking by themselves.
Sometimes it works, sometimes it takes a bit longer. Whichever way though, it's a hell of a learning curve for all.