There are a number of consumer survey sites, and most of them reward contributors with nothing more than loyalty scheme points, prize draw entries, or even just a 'thank you'. But your data is valuable - organisations pay survey companies a lot of money to collect it, and it's great that Populus pass some of that on to the people who give their time to complete the surveys.
OK, you'd struggle to make a living by filling in questionnaires, but it's nice to be rewarded, and I certainly put a little more effort into completing Populus surveys as a result.