Have you ever heard the term wage slavery? It is used to compare certain types of low-paid work to slavery. 
I have heard of that. Who is the biggest user of slaves today?
I'm not really sure, but estimates are that as many as 46 million people work as slaves today. 
I wonder if many slaves are still being used in the United States.
I'm pretty sure that must be the case. The State Department says the term human trafficking refers to people who are essentially used to work (including prostitution) against their free will. I know that happens in the US although I have not been exposed to it personally.