I do not disagree, but I don't remember antivax to be visible in pre-facebook internet either, so I only blame it for ~current wave
Pre-facebook, the barrier of entry was high enough that only fairly educated, students, and the nerds were driving the content.
Once the barrier came down and the floodgates of "average intelligence" were opened, it was over.
It was there, and growing for decades. Pre Facebook it was a small niche but don't blame Facebook, that is mostly coincidence