Also, if you have a small group of family and friends who have been scattered to the four winds for the usual reasons, it's a lovely way to be connected to them daily in an asynchronous, casual way. Perhaps you're lucky enough to have everyone you care about in the same time zone, but a lot of us are not that fortunate.
My brother spent a year and a half in Iraq. We used facebook, google chat, or even video conferencing to stay in touch. A year and a half is a lot longer than it seems when someone you care about is in harm's way. Social media for soldiers is an awesome thing if used appropriately.