If you look on the Unity License Comparison page, you'll see that both the Free and Pro versions of Unity have .Net Socket Support. This is all that is required to communicate with the Smartfox server, because Smartfox is a server-side technology, and Unity interacts with it by acting as the local client.

Using .Net sockets, you can communicate with any other technology on any platform over the internet or a local network, provided it supports sockets as a communication method.

Socket communication is also supported in unity webplayer builds because it doesn't inherently open any kind of security risk to the user. This is because you always need some kind of socket-aware server application listening for connections for the sockets to be able to do anything, so the security responsibility is in the domain of the application listening for connections.