What keeps Twitter up as a major social network is not %99.99 uptime. It's strong network effects in play.
What will sink Twitter is not %99.8 uptime. It's those network effects fading away.
For example, if people stopped using MySpace, Facebook, etc it was not because of their availability.
Again, I'm not saying this is cool. Im just saying if that extra %0.2 downtime (in short term) costs you 80% of your work force costs, a business manager may consider that acceptable.