I'm actually far more interested in Fossil since it became BSD-licensed, but it's not nearly as widely used as the other three. The biggest projects I know of that use Fossil are SQLite, Lamson, Librelist, and Fossil (natch). Git's major projects are things like the Linux kernel, X, and Rails; Mercurial's include Firefox, OpenJDK, and Xen; and Bazaar's users include Emacs and MySQL. All three of those also have large commercial hosting options that are pretty heavily used (e.g., GitHub, Gitorious, Google Code, Bitbucket, Kiln, Launchpad), whereas Fossil currently lacks anything similar.
I'm not saying Fossil is bad for not being widely used; just that it's definitely not yet as popular as Git, Hg, and Bzr.