Rest assured they are NOT making "nothing". If they are paying you to use their services,
they are certainly making SOME money. Even still, that does NOT necessarily mean that
they are legit or will even pay out. The best thing to do is "proper research". You could
start by Googling them and then also look to see if they have a forum with "real" users.
One of the many downsides to using a service like this is that people will soon learn to
simply ignore your links as many people do not want to sit there and view an ad which
most likely doesn't even relate to them. Yes, many of these types of sites TRY to show
relative ads, however, most fail in the true scheme of it.
Not just that, but many of your visitors will not even see the ads, initially, because of
ad blockers and such. When using an ad blocker, and visiting such a site, you're usually
confronted with the "Please disable your ad blocker before continuing" neon sign before
they can even proceed.
If you take the time and energy you would put into promoting paid links and used it to
write up some decent affiliate review articles or even a few info-products, I can all but
guarantee that your time will have been better spent this way than using a pay to link
service(s).