It depends, and I'm thinking in terms of a common social media user's mindset.

The initial attraction to sign up to Steemit for many people is the payout incentive. I think a large number of Facebook users may sign up and attempt to use Steemit as they would Facebook, posting pics of their day-out with no story/description/inspiration, gym-pics, pointless status updates, hoping to cash in on the upvotes. These posts probably won't generate much interaction nor upvotes, and it will weed out these users as they get bored and leave. Twitter users may have more success, seeing as twitter is full of witty one-liners, and hilarious memes/gifs, but I still think unless the individual has a massive following, they would struggle to see the rewards.

A majority of people may only focus on the cash incentive, and totally disregard the community aspect which is the backbone of Steemit, community interaction creates a snowball effect, and shows other users you are here to also offer value.

It takes little effort to post on twitter or facebook, whereas there is clearly more thought, time and effort put into most blog posts on Steemit, and that extra effort is enough to deter some users as they see their effortless posts return little to none interaction. I think a lot of people will sign up and leave after a short period, the user base will grow, but the number of people posting won't grow nearly as fast. The amount of shitty content will increase, but will eventually be leveled out by people realizing they can't use Steemit in the same way as other social networks.

It will all depend on whether Steemits user base of effortless posts outweighs the good ones. The people who like to see things that don't take a lot of effort to take in may upvote those effortless posts - memes, witty comments, pictures of rich people doing ridiculous things, etc. Eventually Steemit would be overrun with meaningless posts and 'fake news'. Hopefully this never happens.

I also wouldn't be surprised if Facebook/Twitter did as you say, and adopted thier own currency.

Thanks for this post