Yes, obviously. Do you think there aren't any anti-vaxxers with the ability to build a website?
An anti-vax post on a FB newsfeed is quite likely to have counterarguments. An anti-vax website is unlikely to run into such problems.
And going back to the original post: does "vaxxers on FB, anti-vaxxers elsewhere" sound like more polarisation, or less?
The ubiquity of the platform and the ease of sharing massively amplifies the problem.
Yes. Why would they not be?