Let's look at Facebook, or Google.

Would you call these products "effective", today?

For example, searching for reviews on Google finds page upon page upon page of SEO spam. Opening Facebook, I don't see much of my close friends, but I do see distant friends having ugly arguments with strangers.

Oh, and on Facebook, almost every ad I see now is for some sort of medical rip-off, it's simply astonishing how bullshit these ads are. I get ads for tonics that I could drink that would supposedly clear "toxins" from my lungs and liver. You don't need to be an MD to know these things are impossible...

I get endless ads for "non-invasive nerve stimulation", something which legally should have to be a licensed and approved medical device, but none of these are. Looking at the details, many of them are passive (without an active power source) and cannot possibly work; others have power and could be an illegal, unregistered medical device but looking at how they're put together and how little power they use, my guess is that they also simply do nothing.

What's the force, exactly, pushing these for-profit companies to give some of that profit up to attain an acceptable user experience?

In classical economics, it would be competition. But neither Google nor Facebook have any real competition.

The process is called "enshittification", and it is the logical consequences of the laws of economics when there's an absolute and unbreakable monopoly.