With all the downtime and seemingly permanent "Older tweets are temporary unavailable" messages why would anyone want to build on platforms they've created?

Maybe they have good ideas but I think part of the reason people are looking at Cassandra etc is because the high-profile sites using/developing these platforms actually work.

If a shard goes down, writes to it are queued on a Gizzard middleware host. What happens if that Gizzard host is then lost?