The several hours long outage of all online services of the Facebook group on Monday had a fairly simple reason, which apparently was quite difficult to correct. Facebook has now offered an explanation in an official blog post.
https://www.notebookcheck.net/One-single-mistake-caused-the-massive-Facebook-and-WhatsApp-outage-yesterday.569459.0.html
Phone networks also went down in several countries, including EE in the UK. Those have nothing to do with the facebook servers. This, coupled with FB employees being locked out of the building on the day and finding their keycards no longer worked, make me doubt FBs explanation of the events