If a scammer, advertiser, or some other form of spammer can get a payout just 1% of the time, they will be willing to pay much more than the average person posting the average tweet.
If you make everything explicitly transactional, you will be left with only people trying to make a profit.
Penis enlargement spam is worth like $0.00000001 per message. Any number higher than that makes them lose money. The real problem is that nobody will post on a social media network where you have to pay to post.
If you make everything explicitly transactional, you will be left with only people trying to make a profit.