I don't know if I've ever really bought into this. I've yet to hear the story of the individual who lost all social ties because they quit Facebook. I made the decision about a year ago, due to excess wasted time on the site and have yet to face any real negative side effects to doing so.
People were social long before Facebook was around, and can remain so without it. Even with an account, the information you share on Facebook is voluntarily given.
I think what Facebook has done most effectively is given people the illusion that their lives are somehow more social because of it.
It's not as if it's either/or. Clearly it is true that new technologies -- say, for example, the telephone -- affect our social lives. Consequently, abstaining, for whatever reason, from using those technologies will affect your social life. To what degree and in what manner totally depends on the individual involved, the technology itself, the way it is being used, etc.
However, one thing is certain, I think. Some technologies may have a transformative effect: The fact that you could act in manner X to get result Y before the technology gained widespread use does not mean you can continue to do so after it has. I'm sure this was true for the telephone, and I'm pretty sure it's true for Facebook and its replacements.
I think its both. While it may be possible in your group of friends to quit Facebook, it might not be so for other people. Imagine quitting the telephone. People were social long before the telephone was around, and can remain so without it. I know that quitting Facebook wouldn't harm my socialization too much, but quitting the telephone probably would. I can imagine people for whom quitting Facebook would have similar effects as me quitting the telephone.
I know a few people who haven't been invited - or have found out too late - about events because they were organised on Facebook and they're not on it.
Those same people are regularly out of the loop in conversations their friends have on Facebook, which at times makes them feel a little isolated.
I've heard of one person who made a huge social faux pas because they couldn't read about someone's breakup on Facebook.
The same are all true of some people I know who live on LiveJournal.
Facebook is a great tool for socialisation, and because of that a lot of people use it. When your social circles use it heavily and you don't, you miss out. Yes, you can do all that socialisation outside of Facebook, but the fact remains that a lot of people don't, and you can't really choose how your friends socialise.
It's not an absolute. You're unlikely to become completely socially ostracised because you're not on Facebook, but it can certainly make some peoples social lives more difficult not being on it.
There /is/ social pressure for some people to use Facebook.
We need more people making this point. How did we manage to socialise up until a mere four years ago? It's still possible today without Facebook. People who say they can't quit Facebook are like people who say it's too hard to quit smoking. No, you really can. You just don't want to.
That's not strictly true; you have to consider the entire system, not just the individual. It's not like only you picked up this new tech and can therefore, if you will it, drop it. Everyone else in your social circle needs to, as well.
It's the same as asserting that people can communicate without phones, or that they can get news without the internet, the phone or newspapers. Sure, they can, but it's incredibly inefficient, exponentially so when everyone else still does.
(I've still managed to stay away from Facebook, but, then, I don't have many friends.)
People were social long before Facebook was around, and can remain so without it. Even with an account, the information you share on Facebook is voluntarily given.
I think what Facebook has done most effectively is given people the illusion that their lives are somehow more social because of it.