Great questions.
In regards to the Yun, we don't support it yet, essentially because we don't support the Programming-over-Wifi. But anecdotal evidence from other companies we've spoken to seems to show a single-digit usage (usage, not sales).
As for new boards... that's also a very interesting question. It does fluctuate, but not as much as you'd think. The case that stuck out most was actually the case of the Arduino Nano clones with the CH340G chip, which made Nano the 2nd most used board.
New boards tend to have a spike in "apparent interest", if I may call it that, in other words in things like Page Views on pages that have to do with the board, or Newsletter clicks, etc, but the actual usage is more gradual
