Amazon has never released Kindle sales data, but Paulo Santos makes a case that sales have plunged — both for the e-ink models and the Fire:
To put it short. The Kindle eReader has dropped out of bed. It has
fallen beyond the wildest dreams of Amazon.com’s management. They
never told it to the market, but this is reality. I have proof,
and the proof is undeniable. The drop in Kindle eReader sales came
with the introduction of the Kindle Fire, and the cannibalization
has been nothing short of stunning, massive. Why do I say this?
Well, Amazon.com has slowed down its demand for Kindle eReader
e-ink screens to near zero since December 2011, the month after
the Kindle Fire was introduced. Basically at that point Amazon.com
had enough Kindle eReaders’ screens on hand to fill demand for at
least 4 months straight.