Just as there is ambiguity around defining a word as simple as open this conversation hints at an emerging struggle with a shared meaning of annotation. As Remi points out, the traditional view of annotation is as a tool acting in service of reading comprehension. What we are talking about goes beyond that.
While Merriam Webster and other disseminators of meaning can add extra definitions to their dictionaries under the word 'annotation', I wonder if an easier path to meaning making might be through using different words. There's a risk that it only leads to complications and muddying the waters over semantics but is it worth considering? I describe it as uptexting (not sure if I thought of that on my own or borrowed from someone else). I've heard University of Texas' Carl Blyth describe this as Social Reading and am drawn to this term because it captures the community aspect and might be less confusing because it doesn't seek to reappropriate an existing, commonly held definition.