As keen-eyed network engineers had already deduced, Facebook's AS32934 withdrew all BGP routes at around 5PM UTC on 4/10/2021. I had already done a blog post in the past with a layman's summary of what BGP is. In short, BGP is, quite literally, the protocol that makes the internet work. BGP is used to announce routes of how one network can reach another network over the internet.
16:42 UTC: AS32934 Route Withdraw
Using RIPE's BGPlay, we can see AS32934 withdrawing its routes and essentially leaving the no path for IP packets to reach the Facebook network. All of this happens in about 10 minutes, after which the internet has fully converged on the new route announcements and no viable path to Facebook's network was available.
23:00 UTC: AS32934 Announcing Routes
After about 6 hours of downtime, Facebook's border routers started announcing AS32934 again and we can again visualize the internet converging. Internet routers started adding these routes to their routing tables so IP transit towards Facebook was possible again.
For more details about the October 4 outage, Facebook has published a blog post with some technical challenges they had during the outage and how they overcome them.
In any case, it's also a big relief that big companies can also have 6-hour outages :) At least, this time, it was not our fault!