ping

ping.js

  
// `ping` backend from client via websocket

// frontend code
// ping every 60 seconds or so
ping.send({
  "user": userid,
  "time": new Date.now()
});

// server code
ping.on('data', handlePing);

function handlePing(data) {
  // save the ping time in user profile (e.g. last activity)
};

or just save a last activity timestamp on a real activity (e.g. topic created, comment posted, ...)