When you sign some agreement to get an account with a social media company, do you read the terms & conditions? I'm pretty sure almost no-one does. This is not the social media company's fault: if you sign a document you have not read you're a fool (and almost all of us, therefore, are fools).
But there's a more interesting question: what proportion of people who agree to terms & conditions understand them? I think that proportion is approximately zero, for two reasons:
- the terms & conditions are no doubt a complicated mass of legalese which is extremely hard for anyone not familiar with reading such documents to understand, and may even be hard to understand for anyone not familiar with the specific jurisdiction they are written in;
- social media companies feed their users' data to complicated and opaque algorithms, probably including neural networks – it's not clear that anyone, including the people who designed these systems, understands how they work and therefore what is actually done with their users' data.
So I think the question to ask is this: can you consent to something you don't understand, or to something that may not be comprehensible by any human?
There is lots of precedent for considering people not competent to consent to various things: ten-year-olds are not competent to consent to have sex for instance. So the idea that a person may not be able to consent to something is uncontroversial.
On the other hand people regularly do consent to things they do not understand, such as medical procedures: lots of people consent to be treated with drugs the action of which they do not understand and in many cases nobody fully understands. But there are usually extensive safeguards in cases like this: medical ethics means that doctors (should / try to) behave in their patients' best interests, for instance.
So I think the answer is that yes, people can consent to things they do not understand, and even to things that no-one really understands such as the actions of drugs, but they can only do so when there is some kind of enforced ethical code in place which should ensure that what happens is in their best interest as far as that is understood. I think it's really clear that this is not true in the case of social media companies: there's no ethical code in place and the companies do not act in their users' best interests.
This means that no, you can't consent to their terms & conditions, even if you read them.