I don't know of any reliable measurement.
Disto Watch has a counter (giving at least a hint),
some distros (e.g. Debian) has popularity counters
(as Debian developers statistics - but its reliability
is also in doubt),
Download Counters for distros (also under debate) ...
Some polls like: http://ubuntuforums.org/showthread.php?t=1717430
(would be nice for LWN - just an idea:).
But getting real numbers ... may be too hard.