This is weird.
Very weird.
As in, I can still access Facebook with no problems. But when I go on isup.me (aka the site you listed), it says it's down.
Anyone having difficulties accessing Facebook? I'm certainly not having any.
...
Oh wait. Just as I was writing this post, it went down. Great.
...
And back up again. But I will point out Facebook is starting to take longer to load things.
But for me, I guess I can conclude that everything is okay for me right now.
(btw, I think you've posted in the wrong category)