(Why) Is post publication peer review (not) taking off?
Most papers that are published are uninteresting.
The median paper has less than one reader not counting the authors and the reviewers.
The current reviewing system relies on a network of responsibilities that are independent of the author.  The editors have a responsibility to the community as a whole to get papers reviewed properly, and the community gives them esteem for performing this duty.  The reviewers have a responsibility to the editor to actually do reviews, and the editor is usually some influential person (and in any case will probably at some point be the editor for one of the reviewer's papers).
Suppose I have written one of my usual mediocre papers that is among the 70% (my estimate) of papers that are never read.  How would post-publication peer review be organized for it?  If I have to find reviewers myself, then everyone is going to say 'No' or do a bad job, because no one feels any need to impress me and it's not a very interesting paper to read.  Moreover, the best reviewers for my paper are people with a little higher status than me, and they would definitely say 'No' to me, but they might say 'Yes' to an editor.  (Keep in mind that, as the system is set up, some top people in a field do far more reviewing than they write papers, while many people write one write-only paper every several years (or grad students who stop doing research write one paper from their dissertation) and never review.)