Perhaps true in the US but not so in Europe. My 16-year old cousin in Denmark (the worlds most FB connected country in the world, 3M users out of 5M population) told me that it's basically impossible to have a social life without being on Facebook (at her age).
Facebook also caters very much to US culture. E.g. In middle school and high school you move between different classrooms so you make lots of different friends that way. In Denmark you sit with the same 20-30 kids every day for 10 years. It's a very different type of social conditioning.
So - if you're the outlier in the class who isn't connected and the party invites go out on FB, guess what? You have volunteered to get ostracized.
I see where your cousin is coming from by thinking that if she isn't on Facebook, she's ostracized, however people tell me the same thing when trying to get me to sign up.
Thankfully, since I never actively used any social networks as a kid, they never became a crutch for me, and any time there's a party worth going to, I'll know about it either through text, a call, or (what most kids seem to avoid these days) face to face social interactions with my friends.
Of course, signing up for Facebook is completely voluntary in a legal sense. No-one can strong-arm you into creating an account.
My point is that social pressure can often make people do things that they don't really want to do. And sadly, many people do not have the courage to stand up to their peers and tell them no.
It's more common in US culture to do that, and largely encouraged by US societal norms, but that isn't always the case in other cultures. This is based on my experience growing up outside of the US (and also spending time in high school and college in the US).
Facebook also caters very much to US culture. E.g. In middle school and high school you move between different classrooms so you make lots of different friends that way. In Denmark you sit with the same 20-30 kids every day for 10 years. It's a very different type of social conditioning.
So - if you're the outlier in the class who isn't connected and the party invites go out on FB, guess what? You have volunteered to get ostracized.