Sure they might. Large companies with deep pockets subsidize the premise to their own benefit (whatever that happens to be). In your example, let's say instead it's $3 per month for just Facebook services (FB, Instagram, WhatsApp, etc). Comcast partners with Facebook on the effort, in which Facebook pays a few dollars per month to subsidize the user plan.
Facebook currently derives something close to $9 per month from US & Canadian members. They hit 45.6% net income margins last quarter, which is extraordinary. They're set to become the greatest pound for pound profit machine in world history, they can afford to use some of that margin to further lock in their monopoly by purchasing preferential access treatment in numerous ways.
Online services (Google, Facebook, ...) make just a few bucks per user. This is peanuts compared to what broadband costs. The fixed costs alone are tens of dollars per month.
"Called Free Basics, it provided only limited access to the Internet through a suite of websites and services that, unsurprisingly, included Facebook"
https://www.cnet.com/news/why-india-doesnt-want-free-basics/
However that breaks down once you consider the fact that it's not practical at all. Not to mention that providers would have no reason to offer such cheap offerings, either.