I had alwas just assumed Redis released this a while ago and that is why AWS started to support Redis along with Memcache in their clustered ElastiCache product
http://aws.amazon.com/about-aws/whats-new/2013/09/04/amazon-...
Redis Cluster on the other hand orchestrates multiple redis instances to function as one, thereby preventing the need for redis servers that are very large and preventing the need for presharding[1].
[1] http://blog.zawodny.com/2011/02/26/redis-sharding-at-craigsl...
They definitely don't use redis cluster, because redis cluster requires explicit client-side support. That support is only available in a limited number of clients today.
Plus, despite the name and where it's hosted, Twitter never used it in production for Redis. Redis support was just added "because we can."
It probably works for very direct use cases, but it's one of those things that could probably bite you very severely under edge cases, and you'd be left on your own to figure out what happened.
Then there's the massive problem of "everyone" using MongoDB and we know how much pain _that_ sucker causes. Playing popularity contests with software isn't useful. People install software either because of social reasons ("I don't know how anything works, but I hear everybody is using X, so I'm going to use X too!") or because they need the functionality ("redis is AMAZING at 30 different things!").
If install base at well known companies were a valid measure, we'd all be using Windows laptops while talking into our Kin and squirting our Zunes all over each other.