callback is called when all of the others are complete. If you have dependencies, like facebook needs the output of redis and mongo then use async.auto which will automatically run things in parallel and in the order you need them.
Yes, what I was saying is that Node makes that really simple, and PHP, Python, Ruby, etc don't.
That's more of a side effect of having everything be asynchronous though.
Use a library like Async, it makes this really easy.
https://github.com/caolan/async
(assume redis, mongo, facebook, and callback are all functions)
async.parallel([ redis, mongo, facebook], callback);
callback is called when all of the others are complete. If you have dependencies, like facebook needs the output of redis and mongo then use async.auto which will automatically run things in parallel and in the order you need them.