Yes, it is voluntary, but it would be socially alienating not to be on that service. Here's a relevant talk [0] by Moxie Marlinspike talking about that.
[0]: https://www.youtube.com/watch?v=eG0KrT6pBPk&feature=youtu.be...
On the other hand: Do people who care about privacy very much really get along well with people who don't and force you in subtile ways to use such a messaging service? I rather think it's the completely different mindset of these two classes of people that is socially alienating and the message service is just a symptom (I believe I know what I'm talking about since I think I am one of these very privacy-caring people).
For politicians my perception is different: one might be tempted to call them privacy activists, but only for their own privacy (for obvious reasons), but not for passing laws protecting the privacy of the ordinary citizen.
Can you square that circle for me and explain what you're doing to be an activist while eschewing the social sphere?
I don't eschew the social sphere, but I strongly lack charisma/am socially rather clumsy and rather the kind of person who has difficulties forming friendships, thus I'm not very social.
There is a whole lot you can do without one-to-one social face-to-face interaction and engagement.