Xbox didn't go down today...once in awhile for some reason the system has issues with party chat and invites. You can actually check by going to Network settings and then looking at nat type, right under it you will find the info.
This is what it looks like when everything is working.
When it's having issue it will say something else.
What happened today didn't stop online you can still play online it's just that certain games had a tough time inviting/looking for people. So people saying xbl was down today are incorrect. Live rarely goes down where no one can play online.