Though many of today's platforms, such as SurveyMonkey and AYTM, have good demographic targeting, you can end up getting biased data because it's a specific set of people who opt into answering surveys for tiny bits of money or charity. For example, what subset of people will answer a 40 question survey for 50 cents to charity (SurveyMonkey)?
Shameless plug, but I'd take a look at HelloToken -- we're able to distribute survey questions to people in the right context (e.g. a reader of a technology article) by getting users to answer a question for access to an article. In this fashion, we can get people who don't normally answer surveys to answer your questions as well as contextually target your surveys to people who are interested in the space in which you work (optional). We have millions of respondents and can get thousands of responses within hours.