https://www.youtube.com/watch?v=uDaBtqEYNBo
Talk by Ben Smith at Rocky Mountain Ruby 2013. The architecture they came up with allowed for many concurrent users, but also helped them effectively scale their development team.
If I remember correctly, their architecture was similar to SOA, but they used Rails engines instead of completely separate services. One noteworthy aspect was that they maintained a dependency graph of the engines, so you could easily run the test suite of the engine you were currently working on and additionally all of those that you depended on (and recurse up the chain if necessary).
Ultimately their project wasn't a completely unmitigated success. They hit some bumps along the way that forced them to re-architect in some non-trivial ways, but ultimately it suited their needs.