The best replacement for socket.io is sockjs.

The best alternatives to socket.io are rated as following:
★★★★★  sockjs
★★★★★  websocket
★★★★  ajax
★★★★  signalr
★★★  ws
★★★  faye
★★  firebase
★★  atmosphere
★★  primus
★  xmpp
★  engine.io
★  pubnub
  websockets

Your alternative for socket.io is not listed? Suggest!
Speaking about socket.io, one probably shouldn't forget about express.

Other search terms related to socket.io and their ratings are:
★★★★★ express
★★★★ node.js
★★ express.js
You have in mind a companion for socket.io and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of socket.io in all years. Click on graph for more.
socket.io
sockjs
websocket