Artillery is a load-testing tool with native Socket.io support out of the box (and is written in Node.js too).
Why is native Socket.io support important? WebSocket is only one of the underlying transport protocols that Socket.io can use and Socket.io layers its own protocol on top, so while it's possible to test a Socket.io server with something like the WebSocket Plugin for JMeter, it certainly won't be as easy to write your tests as when using something that speaks Socket.io "natively", and your tests won't be as realistic (for example, you will skip protocol negotiation and will be connecting straight to the WebSocket endpoint exposed by Socket.io).
(Disclosure: I am the lead developer of Artillery.)