As far as I know this is not possible. It will depend on what you want to do really (and how). First thing that came to my mind as a alternative is identifying your users by socket.id
and then send them the data accordingly.
Example:
io.sockets.clients().forEach(function (socket) {
if(socket.id == 1337) data = {test: "test1"};
if(socket.id == 404) data = {test: "test2"};
socket.emit('data',data);
});