Social networking sites like Facebook let people keep in touch where and when they don't have immediate contact. And that's great. Despite bizarre comments from people who should know better that using such sites can damage brains or cause cancer (see Ben Goldacre's comments on the subject), I'd suggest the bolstering of people's personal networks is a good thing. Once upon a time we'd all meet in the village shop or pub and have a natter. That's not practical for many these days - Facebook et al offer a useful alternative.
However, there does seem to be one danger here. Because of the immediacy and the apparent security of having a password, it's easy to equate such meet ups as being like a private conversation in a locked room. It's not. They are at best semi-public - remember, you will see the comments of friends of friends. It's very easy to write something intended to be private and to be embarrassed to find it has become public.
There have been well publicized cases of employees making comments that their employers found unsuitable - for example the Virgin staff, sacked for calling their customers chavs. Another darker example has recently surfaced. An IT adminstrator has been sacked for breaking into students' Facebook accounts and copying the nude photographs the students had posted of themselves there.
Of course the administrator's actions are reprehensible. But you do have to ask what the students thought they were doing. Unless, of course they would also be in the habit of popping into the local shop or bar naked.