Facebook? They have the motivation. They are getting paid per click. But I don't think they would dare. They would get sued into oblivion if they got caught.
A competitor? They could be trying to burn up my marketing budget. But it seems it would require a bit more technical sophistication than most companies have. And are they really going to pay some shady outfit to do it for them? It seems unlikely?
An ad agency? It will quickly become obvious that you are getting crap return on your spend, so that seems unlikely as well.
Someone else? What is their motivation?