The terms and conditions you agree to on services like Facebook (and it isn’t just Facebook – any decent web service will have a decent set of terms and conditions – it just makes good sense to put these terms and conditions in place on every web site) you contractually limit your options if something goes badly. In the case of Facebook you agree, for example, that you can’t hold Facebook liable for a number of things ranging from bugs to organised hacks. You also consent to the law of a state in the USA and to the jurisdiction of American courts so this presents a practical challenge. If you choose to litigate against Facebook you have to cover the costs of an American legal team in addition to a local legal team as well as the associated costs of conducting litigation in another country.
Of course I am not saying you shouldn’t use Facebook. I use it myself and think it is, on the whole, a fantastic service. What I am saying is that you need to know what you are agreeing to and conduct yourself accordingly.