Facebook runs largest memcached cluster in the world and largest sharded MySQL installation in the world (which they use as a key-value store since it's hard to evolve schema otherwise). So, essentially, quick updates are tremendous amount of duct tape and hand-crafted code to make it work more or less correctly.
I've always wondered why alternative datastores, like MongoDB, get so much good press when a RDBMS like MySQL or PostgreSQL can be used for storing non-relational data in a way that scales tremendously.
There's a video (I don't have the link handy) where a Facebook engineer says they actually had to make code modifications to MySQL to make it do what they want.
They're using the tool in a way that it wasn't designed to be used. Therefore it's probable that that there's an opportunity for specialized software to fill the role better.
