Some sort of security incidence does seem more plausible than a DDoS, although I can't think of what would affect Facebook, Instagram, Tinder, Hipchat, and AIM

Some sort of security incidence does seem more plausible than a DDoS, although I can't think of what would affect Facebook, Instagram, Tinder, Hipchat, and AIM simultaneously. I'm also having a hard time imagining what sort of security incident would result in Facebook deliberately shutting down their web presence, even for a few minutes. And all of the other potential attacks, such as DNS or CDN, seems like it a) wouldn't affect everyone simultaneously, and b) wouldn't even work because sites like Facebook don't have a single point of failure, there's always backups and backups for the backups.