Per-session presence bits.

presence-example.js

var presenceRef = new Firebase('https://mike.firebaseio.com/users/michael/presence');

presenceRef.root().child('.info/connected').on('value', function(s) {
  if (s.val() === true) {
    // we're connected.  Set up presence with a per-session presence bit.
    var ref = presenceRef.push();
    ref.onDisconnect().remove();
    ref.set(true);
  }
});

presenceRef.on('value', function(s) {
  // If there are any active sessions, michael is online.
  if (s.hasChildren()) {
    console.log('michael is online.');
  } else {
    console.log('michael is offline.');
  }
});