This is old and tired, especially when you throw in emotionally charged phrases like "prey on". Does Facebook prey on its users? Around a billion people don't think so.
Facebook does not allow pre-teens on their site precisely because of this law. I do not think that's a bad thing. The law is not perfect, but it is intended to be a tool to stop people from exploiting kids 12 and under. The exploitation isn't necessarily from the site operator, but other users. If Facebook had a bunch of 11 year olds (and now with Graph Search it would be easy to find 11 year olds near you) the main concern would not be that Facebook would somehow take advantage, but that other people on Facebook would.
These same kids cannot go to a PG-13 rated movie without an adult either, I don't find this especially onerous.
With all due respect, that's between a parent and their child. At the very least, the name COPPA is disingenuous and misleading - I'm pretty sure if it was called the Ban Kids From Facebook law, it would not have the same level of public support.
Also, the MPAA rating system is not a legal requirement. AFAIK, there is no law which prevents 13 year olds from watching PG-13 movies, either in theaters or at home: http://www.theclyde.net/TheClyde_Ratings.htm
This is old and tired, especially when you throw in emotionally charged phrases like "prey on". Does Facebook prey on its users? Around a billion people don't think so.