One of the down sides of Facebook (just like almost everything, including Internet, they might have many advantages, but also some disadvantages. Take Internet for example, it has a lot of advantages, but a dark side as well- extreme right groups looking for adepts and Internet provides them with one of the best tools of recruitment, sexual predators that now have the opportunity to "be" how they want on net+ a certain degree of anonymity, etc) is that if you suffer of Depression, Social Anxiety, just going thourgh a bad time in your life, etc- comparing your life with the life of people on Facebook (and forgeting they are showing only the glamorous side of their life) can make you feel even worse. I've unfortunately seen it happen.
The other one is that Facebook might not exactely be forever, but posting without thinking can have very bad results, as some people found out. People can read what you write- so if you post how much you hate your job, employer- and you find yourself without a job because he/she read your rants....well, it is not a very pleasent thing to happen, but it can happen. And your future employers might read it too...if you are a public person, get drunk, post a totally insensitive comment on Facebook (as a guy in my area found out in a quite painful way)....you might end up next day with around 5000 people who now hate you and some of them who wants to get you fired.