FreeBSD on EC2
You can also run FreeBSD on Amazon's EC2: www.daemonology.net/freebsd-on-ec2/
There are quite a number of other 'cloud' providers that also have FreeBSD.
FreeBSD powers more than just WhatsApp as well:
Netflix - Almost all video streamed online comes from FreeBSD Powered servers, making up 33% of all North American internet traffic
Voxer - All servers are powered by FreeBSD and OpenZFS.
Verisign - The .com and .net top level domains are powered by 1/3rd FreeBSD machines
Sony Playstation 4 runs a modified FreeBSD
Yahoo.com - Front page has been powered by FreeBSD since the 90s
Yandex is powered by FreeBSD
The Apache Foundation SVN servers are all powered by FreeBSD
NetApp Data ONTAP storage appliances are powered by FreeBSD
EMS/Isilon storage appliances are powered by FreeBSD
Cisco Ironport, Juniper switches and routers, Citrix Netscaler, McAfee SecureOS Firewall, Dell KACE, Sandvine NPC, Sophos Email gateway, SpectraLogic storage appliances, and The Weather Channel IntelliStar are all powered by FreeBSD.