While there is no dearth of posts related to vaccines, the top 50 Facebook pages ranked by the number of public posts they made about vaccines generated nearly half (46 percent) of the top 10,000 posts for or against vaccinations, as well as 38 percent of the total likes on those posts, from January 2016 to February of this year. The distribution is heavy on the top, particularly for the anti-vax position. Just seven anti-vax pages generated nearly 20 percent of the top 10,000 vaccination posts in this time period: Natural News, Dr. Tenpenny on Vaccines and Current Events, Stop Mandatory Vaccination, March Against Monsanto, J. B. Handley, Erin at Health Nut News, and Revolution for Choice.

Read: How misinfodemics spread disease

Despite panic in cases such as the current measles outbreak in the United States, anti-vaccination activism most likely has been substantially less influential than it sometimes appears to be. As Slate’s Daniel Engber has pointed out, U.S. vaccination rates for measles, for example, have barely budged in recent years: It was 91.5 percent in 2005, and again in 2010, and again in 2014, and again in 2017. What has changed is the (social) media ecosystem. Even if the bulk of Americans aren’t changing their behavior, anti-vaccine talking points are more widely available thanks to the concentration of social activity on a few platforms and the substantial reach of a small number of anti-vax media organs.

Mainstream media outlets and specialty outlets find success in running debunkings of these anti-vax claims. The appearance of a debate generates factional attention, driving the us-versus-them shareability of posts on all sides. Not unlike conservative news outlets that relentlessly cover the Democratic Representative Alexandria Ocasio-Cortez for clicks, “pro-science” pages such as I Fucking Love Science, SciBabe, and The Credible Hulk can attract readers on Facebook by tickling the someone-is-wrong-on-the-internet gland of audiences who already agree about vaccination.

This is not at all to say the sides’ evidence or goals are equivalent. One is spreading misinformation, often to sell a product or service, while the other relies on credible scientific evidence to protect a public-health achievement. But each circuit plays a part in expanding the bubble of attention to the anti-vax position, even though most people don’t buy it.

The crucial factor is the huge audience that just a few social-media platforms have gathered and made targetable through regular posts as well as advertisements. Pinterest recently stopped showing search results for vaccine searches. YouTube told me in a statement that it was “surfacing more authoritative content across our site for people searching for vaccination-related topics, beginning to reduce recommendations of certain anti-vaccination videos and showing information panels with more sources where they can fact check information for themselves.”