I was reading or listening to some commentator the other day and he said more whites and Europeans throughout history were put into slavery than black Africans. The Greeks alone had tons and tons of slaves from every nation. The Greeks even considered a slave as a piece of furniture and not a person. The romans enslaved everyone too, they did not care about nationality.
Throughout all of humanity, slavery was very common. All civilizations had slaves, the exception is the modern European ban on slavery, that eventually was exported throughout most of the developed world.
Asides from the American plantation system, slavery was not focused on race. Anyone could become a slave, despite race, religion, place of birth, etc.
I doubt that Europeans were used more as slaves than Africans. In part, because in Europe slavery was replaced by serfdom.
And although serfdom imposes a lot of restrictions on the serf, it's not as restrictive as slavery.
Another reason is that several African kingdoms were very keen on slavery. So much, that slaves were a common commodity of trade.
Regarding the Atlantic slave trade, it's important to notice, that there were many kingdoms in Africa, each with it's own policy towards slave trade.
Some kingdoms actively pursued the slave trade. for example, the Kingdom of Congo had a treaty with the kingdom of Portugal for trading slaves. This made both kingdoms very rich.
Other African kingdoms, refused to participate on the Atlantic slave trade out of ethical concerns.
Probably the race that was the most affected by slavery were Asians, simply because of the immense population that it always had.