One idea is to have paid private messaging. Pre-approved users (friends and family) would be able to contact you for free; but an unsolicited message, presumably from an advertiser, would require them to directly pay you a fee.
Each user would set their own fee on their mailbox, dependent on how much they value their time vs potential to earn $$.
This is an idea from the attention economy ethos. I'm not sure if it would be a fit for Steemit. But it's an interesting idea, I hope to see someone try.
Interesting concept indeed. This would radically change the way brand managers need to operate. Would indeed be very interesting to see how such implementing will play out. Wondering how the users will view an implementation like that. I can imagine many users may give access for free when they get something in return, maybe a chance to win something. Personally I would want to see 'advertising' not in my face, but somewhere in a separate channel and if I want to engage with a brand, I go to that channel. Think of it like I blog on Steemit and go to FB to engage with brands. And when integrated with Steemit, I go to 'Brand channel' but their push messages are not in my home channel, even when I follow them.