No more than a Ford dealer buying a billboard outside a Chevy dealership. It's not anti-competitive to advertise in the same place your competition advertises.
The issue with Facebook doing this is the fact that they're a huge company that is buying these ads where the smaller company cannot.
It is illegal and anticompetitive to be a huge company using enormous resources to squash competition for smaller firms. There are many ways in which this is done. Not all of them are illegal, depending on how you look at it.
My opinion: Buying marketshare is usually bad for the user.
At the very least, they shouldn't be allowed to advertise for the words "Local Craft Soda (tm)" -- especially if competitor's name is what they are buying.
It is illegal and anticompetitive to be a huge company using enormous resources to squash competition for smaller firms. There are many ways in which this is done. Not all of them are illegal, depending on how you look at it.
My opinion: Buying marketshare is usually bad for the user.