There's also Del.icio.us as a collection/curation platform somewhere between Twitter and Tumblr.
Anyway, I think the author forgot about delicious and flickr, which started in the early 2000's peaking in media hype around 2005, which sort of changes his graph and thesis. It's no longer a nice progression of up and to the right. He just picked out examples that supported his view, and ignore the others that don't.