A quick test recently confirmed that when a customer borrows your ebook and reads it through the Kindle Unlimited program, that counts as a sale the same way as if they had purchased your Kindle book directly. To demonstrate this here are is the salesrank data for the test book:
As you can now see in the charts provided directly from Amazon, there were no sales during this time of the digital edition, but there were KU page reads:
In the end this reflects a basic rule for Amazon: If it generates revenue for Amazon and/or the publisher, it will affect paid salesrank. Books on free promotion generate no revenue and thus return no salesrank data (their is a separate, private salesrank for free Kindle titles visible only on Amazon directly). This also opens up another avenue for authors promoting their book launches.
Recruit Kindle Unlimited subscribers to borrow and read your Kindle book on launch day to boost salesrank and visibility.
Special thanks to author Steven R. for his help in this test.