It's only actually completely voluntary if you have infinite money and no issue (and no repercussion from) completely dropping whole social and professional circles out of your life.
And that still requires the network/system is honest about it[0], when Google automagically creates a G+ account for you when you sign up for (supposedly) unrelated services or forces you to create one to access other content, "voluntary" is really debatable. Again, unless you have no issue shedding whole social circles instantly.
[0] http://arstechnica.com/gadgets/news/2012/01/google-doubles-p...
You only have to pick up the phone instead of using the computer.
[edit]
G+ doesn't change the surveillance situation at all. I have a G+ account and I don't do anything with it. Google doesn't have any additional information.
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.
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.
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.)
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.
You are not forced to share anything. If you want to use Facebook as strictly a messaging platform, there is no negative effect for doing so.
Characterizing it as "surveillance" is RMS's typical good-natured extremism.
And with the "Like" buttons you /are/ forced to share your browsing history (no, being able to circumnavigate it by blocking it doesn't count)
Also, singling out Facebook for what every advertiser cookie has done since the mid 90's seems a bit silly.
Although employment and social circles DO limit your range of actions, you accept the tradeoff voluntarily in exchange for benefits. You can find a new job or people to associate with if the benefits do not justify the cost, without your employer or friends being able to legally threaten you with violence. On the other hand if you refuse to allow surveillance or investigation by the law, they are authorized to use as much force as is necessary to seize your property, imprison you, or execute you. It's completely voluntary in that regard.
"Voluntary" can be viewed as an antonym for "coerced", where coercion implies an underlying threat of violence. Forced denial of an individuals life, liberty, and property can be construed as violence or coercion. Such properties do not presuppose the presence of another person or governing body, so may be considered intrinsic to the individual. However, employers or friends denying the deliverance of their property or empathy to you cannot be construed as violence or coercion. Such exchanges presuppose the presence of multiple people, and therefore cannot be considered the denial of a property intrinsic to one individual.
Imagine I start a FREE postal delivery service for writing letters to your friends. But when your letter shows up at the destination, it is full of advertisements that I hand picked for your friend based on the content of your correspondence. Oh yeah, also the police asked for a copy of the letter. Oh yeah, and we also showed the letter to a bunch of your other friends because we changed our delivery rules and figured you wanted to opt-in. Didn't think you would mind.
But you can't complain. The service was free and you signed up voluntarily.
I'm neither pro / con on the Facebook thing. I never know when the real world rules should apply to the electrons. I think we are just making it up as we go.
My personal email. My work email (can infer where I work from this). Many of the people whom I know who searched for me on facebook. Highschool friend links (can infer my highschool and possibly even age from this). I know for a fact that they have photos of me tagged with my name.
I didn't authorize any of this, yet there it is. Never before has it been so easy to aggregate this sort of information, or perhaps even possible.