One of the many valuable things that come out of the Linux Foundation is an annual review of Linux kernel development. It's just released the 2013 edition (freely available  upon registration), and the news is resoundingly good. Here are the key points .
Nearly 10,000 developers from more than 1,000 companies have contributed to the Linux kernel since tracking began in 2005. Just since the last report, more than 1,100 developers from 225 companies have contributed to the kernel. In fact, more developers and companies are contributing to Linux than ever before with Linux kernel 3.10 seeing the most developer contributions ever.
Read more at Computerworld UK .