Answer: Then Twitter goes down, then Livejournal, then Blogger (briefly), then YouTube (briefly), then LinkedIn (briefer than Facebook and Twitter).
I think part of this is user migration, but probably a lot of it was that one of the big Content Data Networks (CDNs) may have gone down. I don’t have proof. I was watching with DownRightNow.com, which uses Twitter for some of its metrics, so I likely don’t have the complete picture. But I also know that my blogs (hosted on Dreamhost) were also down, so it was something vast enough that storage/operations on big and little were affected.
It also might have something to do with the death of Net Neutrality. But it’s hard to tell, given all these secret, private networks that all our traffic navigates.