We started out with fanout on write but it was tricky. Fanout on write can get incredibly expensive. Customers would implement things like featured accounts, or

We started out with fanout on write but it was tricky. Fanout on write can get incredibly expensive. Customers would implement things like featured accounts, or topics that you can follow. Basically feeds that are followed by a significant percentage of your userbase and update all the time. Companies like Twitter and Instagram control their product and can avoid some of these pitfalls. A full fanout on write approach means you need to enforce certain product limitations, have a large budget and accept that feed propagation is not always as fast.