FreeBSD is or at least was capable of higher concurrent connections per server, which is presumably why Whatsapp uses it. FreeBSD also has a reputation for perf

FreeBSD is or at least was capable of higher concurrent connections per server, which is presumably why Whatsapp uses it. FreeBSD also has a reputation for performing more consistently and being more responsive under duress. These might be important attributes for a social network. When comparing between CentOS and Debian, I'd say Debian's apt system is superior to the RedHat family's rpm system, both in terms of usability and when building your own packages. If you plan to run commercial software however, most are made for use on RedHat although that's starting to change with Ubuntu also being a target nowadays. So in general, I'd say go CentOS for commercial software, or Debian for open source software.