> On the other hand Linux is far more popular so I wouldn't be surprised if ZFS on Linux already clocks more hours of usage than on BSD.

Yes and no. Few people, even among the Linux crowd, are aware it exists or feel like trusting it with important data sets/jobs. Those few are the most likely to be equally at ease running FreeBSD or Illumos (or Solaris), where the damn thing is known to work really well.