First off, you would have to define what you classify as "mainstream".
As an example, if you're a mailer in "mainstream" you can sit around making $500-2000.00+/per day day from your undies hitting send and loading up offers all day. There are entire affiliate programs set up around this, pushing many of the most recognized brand names you see on TV and infomercials.
While you must be CAN-SPAM compliant, have good data, IP's and resources available to some degree, it does not take a long learning curve to apply what you know an adult affiliate marketer, and switch over to mainstream affiliate programs and mailing.
If you have a traditional mainstream sort of site, I am sure they are impacted by discretionary income, access to credit, social networks, and growing number of online options the same as the adult industry. That being said, even in mainstream it will come down to the interaction with your members/mailing list/etc..
