Mainstream is a lot harder to do compared to adult. In mainstream you need to work your ass off for little profit. This holds true especially after manual SEO is becoming lesser and lesser relevant after recent Panda updates. The Panda updates also kicked out most affiliate sites and internet marketing is that attractive unless you have your own product and lots of varied promotion methods. In adult, I can leave an adult site for years yet it will get good enough traffic and adult sites also rise up faster with a lot lesser investment as a lot more people look for porn compared to adult niches. I have had sites in the past which have moved to thousands of pageviews a day within days of launching the site including huge SE traffic. The same does not hold true for mainstream and you need to work hard with quality content to get the visitors to the website. Even then you do not know click rate or conversion rate. So, getting traffic in mainstream is far more difficult than adult and that goes without saying.
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