I'm not sure that usage indicates even rough numbers of hackers. Hacking OS code is a bit more daunting than hacking RoR code (at least in popular opinion), therefore I would think that more users of RoR are likely to hack on it.
That said, Linux is a huge project with tons of contributors.
RoR: Large number of contributors with relatively small patches, few full time contributors who are paid just to work on RoR.
Linux: Possibly smaller number of overall contributors but supplying more patches each, and more patches in total. More contributors who are paid to work on the kernel on a full or part time basis.
The numbers for the kernel are well known (and they are hard to beat for any open source projects):
https://lwn.net/Articles/395961/ (for every version ~1.1k devs and around 10k changesets)
Indeed. I don't recall the exact numbers, though I've heard both Linux and RoR numbers, I think that Linux has factors more developers committing to it than RoR.
Linux is, perhaps, the most rapidly changing piece of software in existence
That said, Linux is a huge project with tons of contributors.