It's the same as any other online business that you start. Profitability depends on a lot of factors. My former boss had a friend that started a membership porn site and it turned into a million dollar business within a year. This was not simply from a build it and profit from it strategy, but took a lot of work to make it happen. Once it was up and running, it was more on an autopilot system. This, however, is true of any online business--porn or not.
Yes but it depends on the business model of the site, like if you are thinking about membership porn site then it can work well without getting much traffic but if you are thinking about free then first you have to spend money of powerful servers then you can earn some bucks from that high traffic. But yes most of the porn sites connected with each other via links and images, so you have to be enter in that network.
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If we keep porn a side, the success or failure or any website depends on your offerings mainly If your visitors find something interesting/useful on your site then you will not only get some regular users but also referrals I have heard from someone that "Its porn where the real money is" so, i wonder its a Good business to jump into & you will learn with time