SystemD is officially the init system for Ubuntu now. Since Debian Project announce they gonna be using SystemD and remove Upstart, Ubuntu is the first Debian family that support this movement.
This is SystemD Init Daemon profile in Ubuntu 15.04 and later.
This has recently changed in Vivid Vervet.
Vivid Vervet (using Systemd by default):
Systemd runs with PID 1 as /sbin/init.
Upstart runs with PID 1 as /sbin/upstart.
Prior versions (using Upstart by default):
Upstart runs with PID 1 as /sbin/init.
Systemd runs with PID 1 as /lib/systemd/systemd.