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...