What is a Web Platform?
On-line platforms can be combinations of web applications runtime libraries and/or environments including various frameworks like Facebook, IBM WebSphere, Microsoft Web Platform Installer, OpenSocial, Oracle WebLogic, WordPress, among others including frameworks for the fediverse. These platforms are not operating systems since they lack a kernel or hardware support, but they have logic similar to that of an OS.
These platforms run on particular web servers (software that serves web applications), which run specific operating systems. These web servers are usually based on and/or coded for Apache or IIS.
As such, most of these operating systems are usually based on Unix like Linux or BSD and Windows servers.