If the only way your friends reach out to you is FB, then they're probably not so much of friends. My real friends know my email, phone, IM, etc. There're plenty of ways to get in touch, if you value the person you're trying to reach.
I use email/phone if I want to do something with someone (mostly phone). There may be events like a house warming party that only gets posted to FB, but somehow that tends to reach me through someone, and if they don't, well.. not much is lost to be honest.
To be fair, most of the people I hang out with aren't that into Facebook/other or even if they are, they know I'm not there so they reach out to me/my wife over the phone.
I'm in my early 30's if it it counts for anything.
Sorta me too, except I'm in my late 50s. I'm not really friends with "a group." I'm friends with individuals, many of whom know each other or don't, and they contact me.
It's a solved problem, since before the web, and even before the internet.
Yeah, I think that may be the main difference from folks that feel disconnected when they are off FB. I have relationships with individuals (or sometimes couples), but never with 'groups'. Probably the most group thing I do is people that play tennis where I do, but again, I have their phones and whenever I want a pick up game, I just do a few calls. I love leaving my laptop/smartphone behind and just go enjoy doing something without the fear of missing something.