[jaywalk]

Obviously not possible to check right now to provide proof, but I feel quite confident in saying that Facebook does not use Let's Encrypt. It's also clearly not an SSL issue.

[gprasanth]

You're right. Fb doesn't seem to be using letsencrypt.

https://crt.sh/?q=facebook.com

On a side note, the amount of phishing sites using letsencrypt and having a domain similar to facebook.com is quite appalling.