The critical investments Google made was paying Apple for search traffic and building their own smartphone-centric hardware ecosystem (pixel, home, tv etc). Android is too indirect. Regulation and OEMs like Samsung can prevent Google from getting the capture over users at the prices they want. Chrome is meaningless outside of ChromeOS / Chromebook.

Apple's demonetization was painful financially but the platform losing steam is a deeper and different problem that started before that event.

Facebook's and Google's customers are advertisers, not end-users, no?

You're not wrong that Apple knee-capped them. The thing that made their stock price dive was all of the applause.

Apple may have been a factor but I think the bigger issue is FB's growth narrative got weak and their valuation was based on expectations of growth. The new story is that they are no longer attracting young, affluent users that as a cohort represent the future ideal advertising target and cash cow for the company. Repeatedly other platforms (Instagram, WhatsApp, TikTok) have shown they can grow rapidly while FB's in-house copies keep failing. More recently antitrust concerns have clipped the wings on FB's primary defense against external competition (early acquisition). Their big bet on the metaverse seems like a big boondoggle that doesn't have legs. Losing cross-app spyware may hurt FB but I don't think it's the biggest reason why the public market have lost faith in FB's growth prospects. They still trade at a PE of around 11 so it's not all doom and gloom.