If you suspect that Facebook's servers are down, you can check sites like DownDetector, IsItDownRightNow, or any other service that keeps track of reported outages for popular websites like Facebook. If you get a message that says “user reports indicate problems at Facebook” or “Facebook is DOWN for everyone”, it's a clear indication that the Facebook servers are down. You can also search Twitter for #facebookdown to see if other people are having the same problem.