Also, it's possible the concept of 'trending now' may not impact user privacy - the whole point of Silk is that pages are partly pre-rendered on Amazon's server

Also, it's possible the concept of 'trending now' may not impact user privacy - the whole point of Silk is that pages are partly pre-rendered on Amazon's servers, so it could be implemented as simply as seeing which parts of the cache are being hit frequently. I don't really know much about how Silk is actually implemented, but it seems like all we know so far is that they're doing at least as much tracking as your ISP already is (and frankly, I find that much more concerning).