The original survey results have been updated (the original is below). You can see the full results at http://theracedirector.com/RaceDirectorFiles/RegistrationSiteSurveyResults.pdf.
RunScore (used to score about half of the finishers in the races in the US and a very popular choice of Timers) did a survey of their users. They received 70 responses in 24 hours of the registration platforms that they used (multiple answers were allowed). Given that RunSignUp is so comparatively new to the other platforms, we were surprised by our close second place finish to Active.com who has been the real leader in this market for over a dozen years.