[loup-vaillant]

Even if RSS was popular, how could advertising companies like Google and Facebook make money off of it?

Those companies have no incentive that I know of to give their user a reliable (a.k.a non-personalised) feed that let them chose whether they'll get to click through or not. No, they want you to click through first, so you get to see the ads before you even start doom scrolling.

[breischl]

Google had a reader app, in fact the market-dominant reader app. They could sell advertising next to it, like they do with everything else (probably did, I'm not sure). They also owned a publishing platform (Blogger) and an RSS metrics platform (FeedBurner) that had paid tiers. So they actually did monetize it. Just not enough to bother with.

Facebook for a while was trying to be the commenting system for blogs. Which is not exactly RSS, but they were in an adjacent space and using it to drive traffic.

And yet it still died, or rather has slowly faded away back to the niche market that I originally mentioned.