I'm never sure if this is a generational thing or just a who-you-happen-to-know thing, but the concept of being an "outcast" because of not using Facebook just sounds ludicrous to me. Your friends wouldn't IM you or email you or text you or call you or talk to you in person if you weren't on Facebook?
I have one friend who doesn't use Facebook (for reasons unrelated to privacy), and it's really quite frustrating. If someone wants to hang out, they send a group message on Facebook. Then someone has to be responsible for communicating to my "outcast" friend via some other medium about the event. This becomes a pain in the ass when trying to schedule things.
I personally would love to use email, but people just refuse to do it. Everyone wants to use Facebook because everyone is on it. Believe me, I've tried to get people to switch to email. But it always reverts back to Facebook.
For some (many?) people, these steps are followed:
1. Plan event (e.g. party)
2. Invite people on Facebook
It’s massively easier than texting/IMing people. Yeah, you will do it for people you really care about, but you can easily miss out if you aren’t on FB.