Thanks for suggesting my website Blair. As I responded to this prompt in another forum, however, in point of fact there are no reliable demographic data for this type of thing.
While USABDA can tell you how many registered amateurs there are, but then there are probably 2-6 times as many people who are more casual regarding their participation (mind you, not necessarily regarding their dancing) that do not register. Similarly, the NDCA can say how many registered pros they have, but numerous small scale and local "teachers" are not within this structure.
Tango, swing, and salsa are even less "centralized," so any types of numbers for them are that much harder to come by -- and less representative.
Finally, perhaps the largest obstacle to such data is the lack of discrete threshold, i.e., when does someone "count" as a dancer? When they have had "x" # of lessons? When they go out "x" amount of time/month? Etc.
http://www.geocities.com/_ynot_/Home.html ~ Studying the culture and community of ballroom and dancesport (and salsa)!