Facebook is a monopoly? That’s absurd. Google also isn’t a monopoly. A monopoly doesn’t mean “big with lots of users and few competitors.” A monopoly is when a company is the exclusive provider of a service or a trade.
Bing, Duck Duck Go, Fastmail, Protonmail, iCloud, Vimeo
Twitter, Ello, Signal, 500px
It isn’t like Google and Facebook control access to steel thus making it impossible to build a new product.
So yes, you can just not use the service. I don’t use Facebook, Insta or What’s App anymore. No big deal for me at all; I just asked actual friends on Facebook for their email addresses. I share photos with iPhoto shared galleries, I use iMessage or Signal, I use Twitter.
Suggesting that we are obligated to accept terms and conditions is nonsense.