There is simply no reliable data on human trafficking. Period. But it definitely exists in many regions of the U.S. Interestingly, part of the problem is the sensationalistic way in which human trafficking is portrayed in the media and to law enforcement.
First, human trafficking does not have to be sex trafficking. Earlier this year, my wife got T-visas (special visas for victims of human trafficking) for several Thai men who were forced to work in agriculture and construction at rates far lower than they were originally promised. This was accomplished through debt bondage, taking away the workers' passports, subtle physical threats, etc. The case has received some attention in various media outlets and it's frequently been portrayed as "Modern Day Slavery!" or something similarly alarmist.
Even though the facts of the case fell well within the meaning of the trafficking statute, there was a lot of reluctance by the feds to investigate because it didn't fit their idea of what human trafficking was: My wife's clients weren't sex slaves; they being forced to labor behind barbed wire or anything like that.
Second, law enforcement doesn't know how to identify cases of human trafficking. Again, part of the problem is that the sensationalism and cliches surrounding human trafficking make it seem rare. So when the cops raid a brothel staffed by a bunch of illegal immigrants, they usually just arrest everyone and deport them. People who are victims of human trafficking usually don't know they're victims of human trafficking and no one asks them the questions to find out if they are.
...So no one really knows how pervasive human trafficking is. It's clear that it exists in more than a few isolated cases, but the 50,000/yr. to 100,000/yr. numbers are probably blown out of proportion.
-WD