Is it in maintenance mode? Chris Mason (the main btrfs developer) works for Facebook now and Facebook uses btrfs on "millions of servers" (source: https://code.fb.com/open-source/linux/).
However, I'm pretty sure if I boiled the commit history down for the kernel, most of Facebook's commits would be Flashcache, and not BTRFS.
What I forgot, though, is SuSE supports it as their enterprise FS of choice, which is maybe more important than Facebook.
If you work some place that doesn't have that kind of luxury, perhaps a little extra conservatism is warranted before you go deploying across "millions of servers".