People don't make plans anymore, make fliers for their parties, etc. So if you decide that you don't want to participate in social networks online, it essentially means that you're opting out of society offline as well: you simply won't be included in those social plans.In my experience, this simply isn't true (I'm not on FB). If your friends do not value your presence enough to contact you via some method other than Facebook, they're not worth having, and most public events also inform people via other methods (email, websites, twitter etc). I would draw a distinction between closed social networks (which require you to sign up and agree to a degree of data-mining just to view content), and open social networks, which act as a broadcast medium. I'm not in your obligatory network effect, and don't want to be; it really hasn't affected my life in the slightest, so yes we do have a choice, and it's really easy and pain-free to say no to FB.
Saying you must be on FB for your life to function is really a post-hoc rationalisation for a choice already made, and as FB becomes more user-hostile while catering to its advertisers, we'll see more people coming up with this excuse for continuing to participate, but the real reason most people are there is simply because it's what everyone they know is doing and through inertia, not because it is necessary in some fundamental way. I've been reading more and more people talking about giving it up recently though.
It's quite possible to have a happy and fulfilling social life without FB or other closed social networks, particularly as you get older and rely on your established network of friends. Email, texts, phones, blogs, websites etc. all existed before the FB ghetto, and will exist afterward. I'm not convinced there is a future for an exclusive social network like FB, which tries to force you to participate and be tracked (even unwillingly) as the cost of interacting with your 'friends' online - that's just too high a cost for people to pay in the long run for simple online interactions which can be done elsewhere on the open web. If FB was open and not intent on selling your data and your life, it would be more attractive, but as it is not, it's not worth joining IMHO.