I don't know the specifics of this outage, but that same website also shows that traffic from Steem users is not that special, Byteball got a lot more bounding when Real Name Attestation bot went viral in end of May thanks to Venezuelan users.

Hubs are for relaying chat messages and notifications and by default, all wallets use the official Hub. You have a choice to pick alternative Hub, but you don't have to. That might be the reason why there is so few of them.