Is there extra communication generated by Socket.IO?
The ping you see sent every few seconds when using Socket.IO is the XMPP online/offline status notification and it is counted as a message. These notifications are propagated to other subscribers (chat room participants) present on a channel.
For practical reasons, it is advised to use Socket.IO when the number of present subscribers (chat room size) is relatively small. You can save on a lot of messages by disabling user presence join/leave events.
NOTE: consider using the presence feature instead of the PubNub socket.io library to avoid the above side effects.