This is how it should be done, ideally. Facebook and similar services behave unilaterally, changing their agreements at will while keeping them dense and obscur

This is how it should be done, ideally. Facebook and similar services behave unilaterally, changing their agreements at will while keeping them dense and obscure, and intentionally make it impossible to communicate with them. Therefore, it's reasonable and valid to simply type into whatever text input Facebook provides "I hereby alter our agreement in the following way" and this should be considered valid.