Linux contributions are not the best example, since it's GPL, and you pretty much have to contribute back to get your changes mainlined, and end-users can request your changes at any time.
Now, if they contributed back to FreeBSD, that would be meaningful, since they don't have to.
Amazon doesn't distribute Linux on server hardware (just consumer hardware like the Kindle) so they don't have to give back for server aspects of Linux like KVM, yet in the Linux kernel code, the Amazon employees are mostly submitting patches for things like KVM, not for Kindle hardware support.
It would surprise me if Amazon use FreeBSD, I thought they use Xen & Linux KVM exclusively?
It's truly awesome that I can send an email to Amazon
saying "we're seeing an odd performance issue here" and
get back "here's a FreeBSD kernel patch I just wrote which
provides a 10% performance boost".
And people claim that Amazon never contributes back to
open source...
Now, if they contributed back to FreeBSD, that would be meaningful, since they don't have to.