Seriously? It's monopoly over the ideas and culture of our society. It's monopoly over the speech that the citizens consume. The culture and ideas that people consume on daily basis obviously affect their actions and ideology.

And yes it is a monopoly. You can tell me that I can go in a town square and spread my ideas if I don't like Facebook, but we all know that means my ideas are effectively unheard, when compared to facebook being able to instantly reach millions of people. You can tell me I can create my own Facebook if I don't like it, which is such an insincere comment, as it's prohibitively expensive to do so, and even if I did, we've decided as a society that it's also fine to then just censor, ban, shut down any system that promotes competing ideas.

But honestly I don't know how you can say with a straight face that facebook/twitter/etc have no bearing on your life. They massively define and shape the society you live in, to a point that has never even remotely existed in all of past human civilziation.