Overseas Exile has a very good post up about U.S. emigration (Americans leaving the U.S.) He is arguing that what was once a phenomenon almost entirely about "pull" (the attraction of other countries) it is now also about "push."
"Emigration for US expats used to be about adventure, love, or a new job abroad. More and more it seems to be about Americans trying to get out."
I think he's right though it is impossible to get any hard data about U.S. emigration because the U.S. doesn't track it nor do they make any effort like the French have to learn more about its citizens living in other countries and their reasons for leaving and for staying abroad - the "sojourners" versus the "settlers." There are more of the latter than Americans in the homeland would like to admit.
In addition the U.S. census does not include American civilians living abroad which means that the United States of America has no idea how many citizens it really has.
Finally in all my research I have only come across one serious study on recent American emigrants, Americans Abroad: A Comparative Study of Emigrants from the United States by Dashefsky et al, which is a great read but since it was published in 1992, it is a bit outdated.
Nonetheless, if you live abroad for an extended period of time and you maintain some contact with places and institutions where the newly arrived American might show up, you can get a feel for trends.
The adventurers are still around but these days you hear more and more about the "push" factors Overseas Exile talks about: high unemployment, shrinking wages, bankruptcies, debt, and less upward mobility. I would add to the list the search for a more family-friendly country (a safer place to raise kids and better school systems), better social safety net, and opportunity (often cited by those headed for Asia).
Should Americans in the homeland be worried about this? That's not for me to say but I think it would be prudent of them to at least look into it.