From what I can understand, once I create a socket, I can then create an array to store it with

userArray[socket.nickname]=socket;

I can then send a message to it with:

  io.sockets.socket(userArray[data.to]).emit('private message', tstamp(), socket.nickname, message);

The basic logic is to store a copy of each socket in an object, identified by nickname. When I want to send a message to that socket, I use the copy of the socket, and send the message via io.sockets.socket(id).emit(). The entire server code is below:

io.sockets.on('connection', function (socket) {
  socket.on('user message', function (msg) {
    socket.broadcast.emit('user message', tstamp(), socket.nickname, msg);
    updateLog('user message', socket.nickname, msg);
  });

   socket.on('private message', function(data) {
        socket.get(data.nickname, function (err, name) {    
        console.log('Chat message by ', name);
        });
        updateLog('private message', socket.nickname, data.message);
        message=data.message;
        io.sockets.socket(userArray[data.to]).emit('private message', tstamp(), socket.nickname, message);
   });

  socket.on('get log', function () {
    updateLog(); // Ensure old entries are cleared out before sending it.
    io.sockets.emit('chat log', log);
  });

  socket.on('nickname', function (nick, fn) {
    var i = 1;
    var orignick = nick;
    while (nicknames[nick]) {
      nick = orignick+i;
      i++;
    }
    fn(nick);

    nicknames[nick] = socket.nickname = nick;
    userArray[socket.nickname]=socket;

    socket.set('nickname', nick, function () { socket.emit('ready'); });

    socket.broadcast.emit('announcement', nick + ' connected');
//    io.sockets.socket(userArray[nick]).emit('newID', 'Your name is: ' + nick, '. Your ID is: '+ userArray[nick]);
    io.sockets.emit('nicknames', nicknames);
  });