No, it is not a direct payment. Yes, it is a transaction.
Yes, there are ways to get around Facebook's ad serving and use the service "for free", in that sense.
Is it possible? Sure.. Is it legal? Probably.. Is it the right thing to do? Probably not..
If you object to the ads being served to you on Facebook, you should probably not use the service at all..
It's more like I want to go to the store, so I invite the store to send me a driver who will use my car to drive me to the store. The store accepts my invitation, but when the driver shows up he wants to invite a bunch of hitchhikers into my car but I politely decline. Meanwhile, I'm driven to the store.
Your bus analogy assumes that Facebook owns and operates my browser. That is incorrect.
But like I said. Analogies get really dumb when you get too deeply into them.