Interesting...
I think it'd be impossible with the mainstream currency system of today. No ads = no revenue for the company.
But if every social media was backed by a cryptocurrency, I think it brings up an interesting dilemma. Right now influencers don't make money from their posts alone. They need to create merch or promote affiliate products to make a living. Even YouTube's preroll ads aren't the driving force behind YouTuber's profits. The point is that the major social media platforms today are just mediums for creators to make a profit and they have to take people off of every platform to make money.
If the influencers have no reason to bring people off a platform then it's in their best interest to be advocates for the platform. Then you have an insane marketing tool for the social network that allows influencers to put in all their efforts to getting people to the platform. You see this already with Instagram and YouTube. So many YouTubers post an Instagram teaser of their latest YouTube video because YouTube is where they make money. (@Adrenalineaddiction is the first Instagram example that comes to mind.)
It's about to come full circle... If Steemit ever gains traction and becomes a power house with hundreds of millions of users then the other platforms will have no choice but to follow suit. Imagine a world where every YouTube, Facebook and Instagram post was used as a marketing tool to draw more people to your ad free Steemit account where you can actually make money but upvoting and commenting on posts. The other networks wouldn't be able to keep up.
So back to your question. Should every social media have a monetary incentive? I don't think so. They're not built that way. They're entertainment mediums for everyday people to connect with others and waste time. Having monetary incentives wouldn't work because that's not what the platforms were built for. Most people on these platforms spend their time as consumers and have no interest in making money off it. They'll always be a place for Ma and Pa to post pics of little Jimmy going to his first day of school and they don't need any money to share that with the world.
That being said, those looking to make money from social media will probably come to Steemit eventually and they'll bring their following with them. This will take the attention away from the other social media platforms causing them to adapt or die. I think the attention they've captured will be used against them as key players push their audience to Steemit. The high level entertainers will be pushed to platforms that pay for it and other platforms will have a more personal, non threatening touch.
It'll be super interesting to see what happens the next few years.
Sorry this comment turned out to be more of a ramble of why I think Steemit is going to win compared to if other social media platforms should use monetary incentives. It took me a while to gather my thoughts and form an opinion :)
There is definitely merit to the idea that Steemit becomes the "home base" for monetization of content on the internet, and then other social networks will feed into it - so if you are a Twitter celebrity (whatever that means lol), you would want to send people over to Steemit whenever you can.
That's a great example of centralized and decentralized platforms co-existing in a healthy eco-system. So yeah, I'm on board! That is an awesome view of how social media can develop for the next 5-10 years.