In addition, where someone with an extremist view might be considered an outgroup, historically their views might have been suppressed among their peers or shutdown. Now they can engage online with thousands if not millions of other people who believe in the same out group ideas, potentially strengthening their view point.
Not only that but those with out-group positions are great for those with agendas and willing to play along. Anti-vaxers are fodder for anyone looking for angry nuts to turn loose on their enemies etc.