Your analogy isn't applicable. What made them slaves is they were not allowed to return to Africa to do whatever. The Foxconn workers are not slaves. They can go back to farming or choose another job.
An employee who serves you is not a slave because he makes less than you.
Most factory workers are characterized by Western media as being poor, uneducated peasants from rural China who migrate to cities as they have little or no opportunities in their home villages.
Now whether this is actually true or not? I don't know.
That's not the analogy I was making. I was saying that there is a certain class of worker who, despite their bad conditions, is probably better off than they would be without the job. Forced or free doesn't factor into it; the question is to what extent supporting their employer is a moral act. Obviously forced labor is worse than free, but there's a continuum of ethical labor practices that both slavery and Foxconn (and any other employer) reside on. I was speaking to the ethical quandary of supporting companies on the low end of that scale, if the options for their employees are something even lower.
An employee who serves you is not a slave because he makes less than you.