The five thousand commits to systemd resulted in the code base becoming slightly smaller -- 387,201 additions and 390,395 lines removed -- in part thanks to removing some features from systemd in 2015. Systemd is up to over 700,000 lines of code (717,649 counted) spread across 2,031 files. There have been commits by 746 authors.
The most prolific systemd authors for 2015 were mostly the usual names associated with the project: Lennart Poettering, Tom Gundersen, Daniel Mack, Zbigniew Jędrzejewski-Szmek, David Herrmann, and Kay Sievers. There were 257 active committers to the project in 2015.
If you are curious about all of these systemd development stats for 2015 and the years prior, you can see my GitStats analytics of the code-base done this weekend via this archive. These are just some numbers for end-of-year reference. If you missed it from last week was the most popular systemd stories of 2015.