Caroline Ghiossi, explains Facebook's spam prevention system, "On Facebook, the most common unacceptable behavior involves some abuse of our communication tools. we've identified certain common patterns of unacceptable behavior. For e.g., we've learned that if someone sends the same message to 50 people not on his or her friend list in the span of an hour, it's usually spam. Similarly, if 75% of friend requests a person sends are ignored, it's very likely that person is annoying others he or she doesn't actually know.
In extreme cases where the behavior continues despite our warnings, we may disable person's account. When this happens, it usually isn't a person's account at all but a fake account or a real account that's been compromised. The compromised accounts are put into a process to give control back to rightful owner. These automated systems don't just prevent spam and other annoyances. They also protect against dangerous websites that damage your computer or try to steal your information. Sometimes, spammers try to hide their malicious links behind URL shorteners like Tiny URL or bit.ly, and in rare cases, we may temporarily block all use of a specific shortener."