I don't think that "popularity = quality" should be presumed at all. In case of the linux distros, I don't have the actual numbers or any other means to truly argue one way or another, though.
Speaking about statistics... Consider 3rd party public mirrors, private mirrors, off-line users, users behind NAT, users on dynamic IPs... I just don't see a way to reliably count users or even update downloads.
Then again, Fedora actually publishes some statistics. While, for example, Ubuntu doesn't. At least the last time I looked, they only mentioned some vague "numbers" in press releases. Has this changed lately?