I don't think there are any solid figures to go by. SuSE and Xandros (etc) can easily tell by the number of sales they rack up for their products, but for the Debians and Mandrakes and Slacks, there's no real way to know how many individual users have downloaded and used (or continued using) their distro.
A site like http://www.distrowatch.com/
might be useful. Check out the "Page Hit Ranking" box on the right-hand column of the webpage. That will at least tell you what kind of interest each distro is attracting.
Maybe someone else on these forums has more solid figures...?