well, we did the questionnaire, we cross checked it with identical hard copy questionnaires our client handed over to audience (in the country we were focusing our research) and we actually found in quite some questions similar results.
Also, we had some open-end questions (serving research purposes as well as to test the “authenticity” of the audience) and we also received a good number of multiple responds. All in all i could say that either Survey Monkey really works or they have created a really really sophisticated software … 
in any case, online surveys are great as we had in seconds all the statistics we needed for our report that otherwise we would need lots of time or to hire a consultant.
if any other has experience with Survey Monkey or other similar services please share it.