I had a blog or two that used Google AdWords circa 2008. I got very close to the $100 limit but never reached it - so Google just kept my $90. I wonder how many people and how much revenue that applies to.
I had the same issue, Google escheated the money to the state of Delaware (where they're incorporated) after a few years though. Consider checking Delaware's unclaimed property site, you may find your money there.
