PageRank is predicated on an assumption that most pages (and thus, most links) are created/curated by humans. This was true when it was invented, but appears to be less likely now.

What gives me pause here is all the anecdotes in this thread about other engines getting results right. If the real answer is "PageRank has been successfully flooded by bots", then everyone would have bad results.

What I suspect, off nearly no evidence, is that Google is using ad tracking to inform a notion of search relevancy. My nearly unjustified belief is that that system is the one being flooded by bots.