>I don't use it for people I keep in contact with in other ways.Seems to me that the "other ways" is the key barrier for most people. The "other ways" works for you; it doesn't for most others.
The typical answer of alternatives to Facebook is "email" and "what about picking up the phone and calling?" etc but those are not low-friction broadcasting mechanisms. You can't post a "status" via email -- unless you create a "mailing list" of friends or carbon-copy email to a bunch of people. That would be a very nerdy way of doing it. That's also a "push" mechanism instead of the "pull" mechanism of others following your newsfeed on Facebook.
Email also doesn't have a built-in calendar mechansim for coordinating meetups/events. Email also isn't great for sharing a set of vacation photos. (Oops, forgot to attach my photos in my previous email. Or, error message, your free email account is limited to 10MB attachment limit, etc)
For typical non-HN readers of 1+ billion people, the altenatives to Facebook using combination of email + geocities web pages + voice calls is way clunkier. As for the other option of using alternative social networking websites such as Google+, the answer has been covered numerous times: "network effects".
To me, the massive use of Facebook by so many is very logical consequence of how everything else that is not Facebook doesn't work as well.
Personally, my favorite method of keeping in touch is email but I recognize that a billion other people don't think like me.