HelpMeVote is a Greek Voting Advice Application that was completed about 480000 times in the period from its official beginning (18/04/2012) until the Election Day (06/05/2012). The number of citizens who have participated in the Greek Parliamentary Elections of May 2012 is 6476818. Thus, if we suppose that all HelpMeVote users have used it only once and that all of them have participated in the elections, then we can estimate that HelpMeVote users are circa 7.4% of those who participated in the elections.
You can download the Statements used for HelpMeVote 2012 and all the files of the study HelpMeVote 2012 from ICPSR.