I wouldn't call a Joke something that gave wings to Google, and thing that still have such an audience, and lot of webmasters talking about it 
But to answer your post. Backlinks jumped you said, what about external links, they jumped too? If yes, maybe that is the reason.
Also some backlinks don't pass PR.
You may have lost some of good backlinks...
Where did you checked your bl? Google doesn't show all of them, try Yahoo...
And for the last, the PR you see, toolbar PR is just a glimpse of true pr and it is not immediate process where you should see results in first update... This is the reason why PR has no such importance anymore, it is not accurate data.
But PR is not a joke :P
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