If you need Facebook to stay in contact with faraway people then you aren't good at keeping in contact.
You don't need facebook to keep in touch with people, but it's incredibly convenient, especially if you have a very wide group of friends that you don't see very often.
If, for example, you know people all across the globe, then you could store e-mail addresses, or real ones, and send the same message to everyone in a weekly update, or something similar.
Using Facebook to see what those friends are doing, and let them know too, is wonderful. A photo here, or a re-affirmation of a person's interests in the form of an interesting link or update keeps people in touch.
Next time I go abroad I will almost certainly see a few friends with whom I would otherwise have lost touch: similarly, a number of friends have visited me here but would not have cared to contact me directly without the background 'knowing' that this indirect communication on Facebook gives.
A lot of things you know about your friends you get from gossip and other mutual friends. Facebook makes up those occasions when you don't have such sources because people are far away. It nicely imitates the way social relationships really work.