I don't know what it is but Xbox Live has some serious issues. It seems like it's always down! Or at least, only semi-functioning. I don't think the PS4 network is any better, though, so maybe it is just hard to keep these networks up at running all the time.
I think that they weren't expecting so many people to use it. This sort of failures happen when there are too many people on the site and the servers can't handle it. I really think that Microsoft has to give this issue a lot more importance then what it has until now.