If you see ads on facebook that have the disclaimer "ads not by facebook" or just advertisements that seem to be in places other than where facebook ads usually are, it might be a sign that your computer is compromised.
I do not know exactly what caused it in this case but from the number of hits this term generates (About 766 results as of this writing) on Google web search, not many people notice this as a problem at all.
I will report back when I have more to share about this issue. Keep your eyes open and be secure on the web. Knowledge is your friend.