Just imagine the gargantuan changes that could be necessary for small tweaks like that. Twitter was built on RoR originally, so it was probably a pretty standard relational model, then they did massive scaling for huge read/write loads, then they did massive scaling for real time features, god knows how hard it would be just to add a foreign key. I really, really, want to take a day and just go through all their engineering blogs after only glimpsing a few..