update I got from Nofish/ZeroMe regarding Hubs:
ZeroNet if full P2P, there is no need of servers for hubs. Everyone who is downloaded or interacted with a hub he/she is also hosting every profile on it. So the hub only go down if every people shut downs it's computer at same time. That's already means hundreds of computers (and the number increasing every day), so in the reality there is no real chance for any downtime.