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.
available.
Definition.
The Office to Monitor and Combat Trafficking in Persons agency of the United States Department of State says that modern slavery', 'trafficking in persons', and 'human trafficking' have been used as umbrella terms for the act of recruiting, harboring, transporting, providing or obtaining a person for compelled labor or commercial sex acts through the use of force, fraud, or coercion". Besides these, a number of different terms are used in the US federal Victims of Trafficking and Violence Protection
Slavery in the 21st century
Contemporary slavery, also known as modern slavery or neo-slavery, refers to institutional slavery that continues to occur in present-day society. Estimates of the number of slaves today range from around 21 million to 46 million, depending on the method used to form the estimate and the definition of slavery being used. The estimated number of slaves is debated, as there is no universally agreed definition of modern slavery, those in slavery are often difficult to identify, and adequate statistics are often not