...And in fact has not worked since at least Opera 51, according to "A Former User" (see below).
My OS is Ubuntu 18.04.4 with all the latest updates installed. I have the latest Opera as of this time (Opera-beta 70.0.3728.59 - I also tried with the non beta, and it has the same behavior). I also tried deleting all the relevant Opera and Facebook cookies and the Opera cache and, as you must have guessed, nothing worked.

By "Facebook comment plugin" I refer to the external Facebook comment section of various websites. In what manner does it not work? Almost exactly how A Former User described here, back in March 2018 :
https://forums.opera.com/topic/25534/facebook-comment-plugin-is-not-working

The relevant section from the Former User's bug report (which was inexplicably locked with no replies at all..) is this :
"Some newspaper sites use this social plugin on their comment section and when I try to comment an article, it shows a popup window where I login to Facebook but after I've logged in it starts a loop that refreshes the page and a blank popup shows and closes for so long as i click previous page."

The slight difference is that my endless loop starts before I even log in the comment section (though I am logged in Facebook itself, with which I have no issues). The loop (of the comment section frame, not of the entire page) starts in all three cases :

1. When I try posting a new (main) comment.
2. When I try replying to another comment.
   and 3. Even when I "like" someone else's comment.

The loop stops only if I reload the entire page or if I close the blank pop-up window very quickly a few times. It is also website agnostic, i.e. the problem is not just with one website (otherwise I would have reported the bug to that website). The Facebook comment section of thee distinct websites I just checked behaves identically. A Former User first dealt with this problem while using Opera 51. I have seen the same issue, consistently, since at least Opera 60 - maybe even earlier. Will it ever be fixed?

@leocg Hello. https://www.politico.com/ and https://www.huffingtonpost.gr/ are two examples of websites I frequently visit (particularly the former). The comment sections behave the same on Opera. I do not see Facebook comments in as many websites as before these days but I do see them occasionally, and I'd like to be able to comment from Opera.

...And in fact has not worked since at least Opera 51, according to "A Former User" (see below).
My OS is Ubuntu 18.04.4 with all the latest updates installed. I have the latest Opera as of this time (Opera-beta 70.0.3728.59 - I also tried with the non beta, and it has the same behavior). I also tried deleting all the relevant Opera and Facebook cookies and the Opera cache and, as you must have guessed, nothing worked.

By "Facebook comment plugin" I refer to the external Facebook comment section of various websites. In what manner does it not work? Almost exactly how A Former User described here, back in March 2018 :
https://forums.opera.com/topic/25534/facebook-comment-plugin-is-not-working

The relevant section from the Former User's bug report (which was inexplicably locked with no replies at all..) is this :
"Some newspaper sites use this social plugin on their comment section and when I try to comment an article, it shows a popup window where I login to Facebook but after I've logged in it starts a loop that refreshes the page and a blank popup shows and closes for so long as i click previous page."

The slight difference is that my endless loop starts before I even log in the comment section (though I am logged in Facebook itself, with which I have no issues). The loop (of the comment section frame, not of the entire page) starts in all three cases :

1. When I try posting a new (main) comment.
2. When I try replying to another comment.
   and 3. Even when I "like" someone else's comment.

The loop stops only if I reload the entire page or if I close the blank pop-up window very quickly a few times. It is also website agnostic, i.e. the problem is not just with one website (otherwise I would have reported the bug to that website). The Facebook comment section of thee distinct websites I just checked behaves identically. A Former User first dealt with this problem while using Opera 51. I have seen the same issue, consistently, since at least Opera 60 - maybe even earlier. Will it ever be fixed?

p.s. I tried posting a comment with the error messages I get from the command line when trying to run Opera-beta 68 under Ubuntu 18.04 Unfortunately, despite how it looked when writing the comment, it appears that comments here are completely "monolithic", i.e. they don't recognize paragraphs and indentations at all. So the comment ended up looking like a block of mess, with the command line output mingled tightly with my own words. Sorry about that, and if the mods wish they may delete it. Thanks.

Opera 68 beta does not load (or crash) under Ubuntu 18.04 Multiple Opera-beta processes (at least 15) show up in the System Monitor and then disappear. I tried running Opera via command line to find out what's going on and I got the following error messages :

command : opera-beta
[6367:6367:0311/131107.487666:ERROR:edid_parser.cc(102)] Too short EDID data: manufacturer id
[6395:6395:0311/131107.865015:ERROR:sandbox_linux.cc(374)] InitializeSandbox() called with multiple threads in process gpu-process.
Illegal instruction (core dumped)

[6414:6414:0311/131109.828483:ERROR:edid_parser.cc(102)] Too short EDID data: manufacturer id
[6452:6452:0311/131110.391573:ERROR:sandbox_linux.cc(374)] InitializeSandbox() called with multiple threads in process gpu-process.
[6471:6471:0311/131112.251042:ERROR:edid_parser.cc(102)] Too short EDID data: manufacturer id
[6509:6509:0311/131112.601456:ERROR:sandbox_linux.cc(374)] InitializeSandbox() called with multiple threads in process gpu-process.
[6529:6529:0311/131114.291698:ERROR:edid_parser.cc(102)] Too short EDID data: manufacturer id
[6568:6568:0311/131114.702953:ERROR:sandbox_linux.cc(374)] InitializeSandbox() called with multiple threads in process gpu-process.
Discarded=1
Discarded=1
Discarded=1
Discarded=1
Discarded=1
Discarded=1
[6570:6573:0311/131144.417640:ERROR:broker_posix.cc(40)] Recvmsg error: Connection reset by peer (104)
Discarded=1
[6512:6516:0311/131145.322575:ERROR:broker_posix.cc(40)] Recvmsg error: Connection reset by peer (104)
Discarded=1
[6454:6458:0311/131145.468192:ERROR:broker_posix.cc(40)] Recvmsg error: Connection reset by peer (104)

For the moment I'll try downgrading to Opera-beta 67.