http://www.pewresearch.org/fact-tank/2016/05/02/q-a-online-nonprobability-polls/

What do you hope the survey world will take away from this study?

Our results suggest that more elaborate adjustments help to improve accuracy and that not weighting your data – a common practice among online sample vendors – is not a great idea. To advance this survey approach, our study suggests that you need adjustments across a large range of variables, rather than the narrow set often used when weighting traditional, probability samples.