p.t.c. sites - a great way of online earning - or - waste of time

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Now a days P.T.C. sites are growing like mushrooms. Every other day you come across to a new one. Many of them are proven scams. But some of them really pay for your clicks. Almost all of them have very low payment rates. In my opinion these rates are so much low that even cost of computer equipment, Internet, electricity used, your time etc is not covered. They only exploit the innocence and needs of the users. They also use tactics to get back the amount earned by the user. I think it is impossible to make a living by working for these sites. Even then why these sites are so popular. What do you think are these a great way or earning online or waste of your time & money.