BSD is an operating system, Linux is a kernel, checkout the FreeBSD / OpenBSD / Darwin sources and you'll find a complete OS in the source tree.
Debian/FreeBSD isn't linux because it doesn't use the Linux kernel. I think there is more question as to wether most Linux distributions are Linux or GNU/Linux than whether Debian/FreeBSD is Linux.
If BSD is an OS then I don't think OSX is a flavour of BSD...
I don't disagree with any of what you're saying there, I just think it's a very grey area where you have multiple kernels and userlands, some of which are fairly easily interchangeable. And yes, most linux distro's are GNU/Linux (or at least something/linux) where Android/Linux is quite distinct.
Debian/FreeBSD isn't linux because it doesn't use the Linux kernel. I think there is more question as to wether most Linux distributions are Linux or GNU/Linux than whether Debian/FreeBSD is Linux.
Hopefully RMS doesn't read this comment.