I'm seeing the icons here at UBBCentral, so the coding in the forum software stock files still should work...
In order to troubleshoot we need basic information:
Forum URL?
UBB.threads version?
Any system changes that you're aware of?
Do you have any modifications to the stock files of your UBB.threads install?
Are you using tracking protection, a privacy setting, script, or plugin in your browser which would stop 3rd party (read: not from your site, such as Facebook in this case) coding from loading?