So here is that hypothesis - that the shift from people using blogs to blog represents the increasing dominance of a Flickr-style paradigm of tagging. Imagine the process of annotating a weblog - if you tag it with 'blogs' it seems clear that you are adding it to a collection of some kind. 'Blogs' is clearly the name of a folder which houses links to weblogs rather than an attempt to describe the weblog itself. But tagging something with the term "blog" suggests quite the opposite - to tag a link 'blog' suggests that I'm attempting to describe the link not as belonging to a bin labelled 'blogs' but simply as a 'blog' in and of itself. It is my conjecture, therefore, that the folder metaphor is losing ground and the keyword one is currently assuming dominance.