As jedberg said, we're heavily invested in Linux on the cloud, which means there's a large cost associated with the move (porting infrastructure software, tools, and testing).

But FreeBSD has been improving on EC2, such as PVHVM support. If the benefit was big enough, it might outweigh the cost. I'd like to do a little FreeBSD EC2 perf testing this year (as I would for any tech that might pay off)...