Diaspora is the crowd-funded open-source decentralized social network built on Rails. Full buzzword compliance: on by default. We have many thousands of active users and they generate a lot of social data. But after nine months of full-time development with MongoDB as our primary storage engine, a few months ago we converted it all to MySQL. Wait...what? Most people are going the other way, dropping Mongo into a project in place of MySQL or PostgreSQL. Plus, conventional wisdom says that social data is ill-suited to a traditional data store. Come hear a story about a large-scale Rails project that tried it both ways. You'll see crisis and redemption, facts and figures, nerds, kittens, ponycorns, and, of course, the secret sauce. Hecklers will be piped to /dev/null.