Oauth Facebook icon is taken from facebookbrand.com which seems to not work for couple of days due tu invalid cert and is marked by antiviruses.

https://about.meta.com/brand/resources/facebook/logo/ should be the page to use probably. 

The icon URL is hardcoded in the service here: https://github.com/moodle/moodle/blob/8a53fe7f6b896e1a963c243a84b803e408941b0c/lib/classes/oauth2/service/facebook.php#L41 . Of course this can be changed by any administrator during configuration.