"As a result, when one types Facebook.com into a web browser, the browser has no idea where to find Facebook.com, and so returns an error page."
Not quite.
Many DoH servers are working fine. DNS isn't a problem for the browser, but it seems to be a problem for Facebook's internal setup. It's like their proxy configuration is 100% reliant on DNS lookups in order to find backends.
The FB content servers are reachable. It is only the Facebook DNS servers that are unreachable.
Don't take my word for it, try for yourself
www.facebook.com 1 IN A 179.60.192.3 (content)
static.facebook.com 1 IN A 157.240.21.16 (content)
a.ns.facebook.com 1 IN A 129.134.30.12 (DNS)
ping -c1 157.240.21.16 |grep -A1 statistics
--- 157.240.21.16 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
ping -c1 179.60.192.3|grep -A1 statistics
--- 179.60.192.3 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
ping -c1 -W2 129.134.30.12 |grep -A1 statistics
--- 129.134.30.12 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms
curl -HUser-Agent --resolve www.facebook.com:443:179.60.192.3 https://www.facebook.com|sed windex.htm
links -dump index.htm
[IMG]
Sorry, something went wrong.
We're working on it and we'll get it fixed as soon as we can.
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grep HTTP index.htm
HTTP/1.1 503 No server is available for the request