How about just don't use their services and boycott their tech stacks at work? I know, it requires some brain use and more than just shouting at the cloud, but it's doable.
It also ignores the reality of the prisoners dilemma, in that a service can be bad for society over all but within that society it is better to use it than to not. Everyone driving around huge cars is a clear example of this. When a scenario like this is identified the correct thing to do is regulate it with law.
Also big companies are easiest to regulate and all the countries regulate them all the time. If you have 100 different TikToks, they threaten democracy even more as the profit incentive push towards extremism in any case.
Frankly, your call to 'critical mass' reminds me of sjw and leftist propaganda.
The same should be true of democracy, but unfortunately, in representative democracy, you only need to convince a few representatives, which does not actually require a critical mass of people agreeing.
And meanwhile, at least one Danish ministry has decided to drop Microsoft Office, and more are likely to follow if that goes well.
So, there is some movement to the right direction, but big changes take a long time...
Boycotts are the one democratic practice that hits a company's bottom line. Just ask Target.