Before I even begin this section, I want to note my general thought that annotations prior to the spread of the internet (and even up until just the last few years, let's say to the birth of Hypothes.is) were generally seen by very few people--like those that wrote them and at best perhaps two or three others unless they were later moved into longer papers, speeches, or other material in another aggregated form. Owen Gingerich's study of this in The Book that Nobody Read is like an extreme case of the spread of annotations from professors to students and I suspect would be an outlier.
However even in the current climate with a more social media based networked set of annotations, they're still not widely used or distributed, though are becoming somewhat more so as the ease of use of the tools and the adoption by smaller groups takes hold. But even given this small growth, by analogy I might say that adoption of digital annotations are at the level of late first century Christianity--it's still small and is far from the spread and acceptance of, say, 17th century Christianity.