Groupon comes to mind as another one, relevant to the propped-up-by-ad-spend part of this story. They IPO'd at a valuation of $12.7 billion in 2011, and have since lost about 2/3 of that market value. However as a post-IPO loss of market value it doesn't implicate the same sorts of things described in this article.
Poor market bet imo, Groupon is great at what it does no matter valuation. I would be very sad seeing it to run in serious issues due to its dropping valuation.
