Few people talk about that in any survey design. People argue that because online survey is self-selected, the sample is not random. Therefore, the result is less reliable. Unfortunately the argument is partly correct. However, although the survey result becomes less reliable because of self-selections, a well designed survey produces the most reliable result we are able to get.
Any marketing researches, no matter online or offline, suffers the same self-selection shortcoming.
Any method to overcome this shortcoming will introduce new bias in the survey. If you ask correct questions, no matter what sample size you use and how you sample the population, you will get a much more accurate result for your decisions. When you want to improve your profit, you conduct a customer satisfaction survey. Or when you want to improve your customer satisfaction, you ask your customers whether your employees are knowledgeable. These are examples of how surveys ask wrong questions.