Where basicaly people are able to complete free offers and surveys and be rewarded for each successfull survey. Once the members reach a minimum in their balance they can get their reward in gift card such as amzon, steam...and of course bitcoin as i know it is possible with tangocard)
As owner it is just a matter to work with companies such as PeautsLab and many others and install theirs offers and surveys in your website (ie, just intalling some modules :p) the market of survesy and reward sites is quite big, there are lot of money to make for both sides, owners and members
There are already a lot of services like that around and i don't think people really take much interest in them as they are too much time consuming and rewards are really small for them and if a services gives quite bigger rewards then they need a small investment as well which mostly people get afraid of doing.
they don't need to invest anything more than their time
there are members earnings like 500$ per month sure it is time consuming, working also is time consuming. in some countries 500$ is 3 times the minimum wage in their country
there are sites with over a million members and more so yes i guess people are interested
the sites you are referring are the bad apples of the industry, junk websites run by people with like 300$ in their pocket, it is called PTC sites wich is different than Gpt sites
PTC websites don't have a revenue others than the investment from members, who are investing to earn more, owners use the money of the next idiot deposit to pay the others, it works with a pyramid scheme, nothing more than use the money deposited from b to pay A
Those "hybrid" sites don't last long more than 4 months