We do not use foreign keys (2016)
When posts like these come up, I'd like to remind people that context matters when making technical decisions. What works for large companies with huge scale (GitHub, Google, Facebook) may not work for you.As a counter point to the linked issue, I operate a few small applications. Foreign-keys (and  constraints in general) are great at ensuring that invalid data doesn't find its way into your database. Yes, they have a performance cost. Yes, they make sharding more difficult. In my experience, at smaller scale the trade-offs are worth it. YMMV