openSUSE and Fedora being the first, I think those are particulary important as Red Hat Linux and SuSE are the most important enterprise distributions out there. RHL 7 is going to have systemd and in all likehood so will SEL 12. This is of course going to lead to Oracle Linux, CentOS, Scientific Linux... to adopt systemd too.
Then there was Mageia, Mandriva, Arch Linux, frugalware, Chakra, NixOS... just from the top of my head.
GENEVI Alliance and The Linux Foundation's Automotive Grade Linux Workgroup both chose systemd as the init system of their IVI platforms. This means top class support for systemd from various embedded system vendors, MontaVista Linux for example.
Mobile operaiting systems like MeeGo, Tizen, Mer and Sailfish have all chosen systemd too.
Then there's the rest of the embedded system market that is less formal but to my understanding peole have been very interested in it. The ?ngstr?m and Yocto for example has support for systemd.
But yeah... no one is forced to use systemd.