You're thinking of the "public" PageRank value previously displayed in Google's browser toolbars. Those are indeed no longer updated. However, there is still some variant of "PageRank" in use internally.

They are probably still using it, but I think they apply spam and duplicate content detection first and also limit PageRank flow to sites of similar topic. Another thing they could do is to nullify the old links for resold websites.

Besides that, time spent on page and social signals such as likes and tweets probably count for more than links. A crappy spam site would be disregarded pretty soon, because people would immediately bounce back to Google and try other sites.