Immigrants from Africa, Where are they?

Another of Joergs' questions relates to the relative absence of African migrants in the current debates:

BTW, I also think it is not accidental that in the context of immigration, Asia gets preferred treatment. Nobody talks about taking in immigrants from Nigeria and the Congo. Germany doesn´t have any specific "identity issues" in this respect - there is a pervasive racism in western society operative here. Believers in the "Bell curve" are strict about which type of immigrant they will accept. As a general rule, you just need to look at the incidence of intermarriage to find out how genuinely free relations between races, peoples and religions are. I would think that with regard to the African-American population there are still very real obstacles to be observed - though I haven´t checked the data
recently.

Here is a link to an interesting paper from Harvard economic historian Jeffrey Williamson, explaining why mass migration out of Africa may well lie in the future. Essentially migration processes are fuelled by the demographic transition at home, and then driven by migrant stock abroad and mimimum wealth requirements. Essentially Africa is still too poor, and with relatively few external migrants to send money home to pay for more. But give it a few years......

Here's the abstract:

Two of the main forces driving European emigration in the late nineteenth century were real wage gaps between sending and receiving regions and demographic booms in the low-wage sending regions (directly augmenting the supply of potential movers as well as indirectly making already-measured employment conditions less attractive). These two features are even more prominent in Africa today, but do or can Africans respond to them with the same elasticity as in the days of 'free' migration? Our new estimates of net migration and labor market performance for the countries of sub-Saharan Africa suggest that exactly the same forces are at work driving African across-border migration today. Rapid growth in the cohort of young potential migrants, population pressure on the resource base, and poor economic performance are the main forces driving African migration. A century ago, more modest demographic forces in Europe were accompanied by strong catching-up economic growth in the low-wage emigrant regions, followed by a slowdown in already-modest demographic growth. Yet, migrations were still mass. In Africa today, economic growth has faltered, its economies have fallen further behind the high-wage OECD leaders, and there is a demographic speed up in the making. Our estimates suggest that the pressure on emigration out of Africa will intensify, manifested in part by a growing demand for entrance into high-wage OECD labor markets.