Cloudflare describes how things looked from their point of view the day that Facebook, along with its other properties, went down. From the Border Gateway Protocol, which defines routing information:
A BGP UPDATE message informs a router of any changes you’ve made to a prefix advertisement or entirely withdraws the prefix. We can clearly see this in the number of updates we received from Facebook when checking our time-series BGP database. Normally this chart is fairly quiet: Facebook doesn’t make a lot of changes to its network minute to minute.
But at around 15:40 UTC we saw a peak of routing changes from Facebook. That’s when the trouble began.