Were you airplaying to your AppleTV and thus the notification was associated with the source device?
Could it have been something in the content itself? For example, were you watching a video perhaps captured from a FB video and stored on YouTube? Then you watch the YouTube version and see the FB badge burned into the video. If so, you could see ANY icons this way: Playboy, Android, Windows, etc. Someone may have just grabbed a bit of video from FB and then made it available on YouTube or another (amateur) source.
Did it look like this (Facebook) icon...
...or different than this?
Try to snap that picture of it next time it happens so we can see what you see, then describe what you are watching, how you were watching (airplay or native), from what streaming service, etc? Also, pause whatever you are watching when it shows, back up 30 seconds and then resume to see if it will repeat at the same point in time (which would imply it is embedded in the video itself).
If you can identify the specific video and time in the video when it shows, we could try watching the very same video on our AppleTVs and see if we see the same. We just need specific details to try to replicate the issue.