If people were willing to pay for things they used, we could get rid of the damned ads. App.net was an interesting experiment in this, but it did not end up taking off (I don't recall the price anymore).
Would definitely bring down botting on these platforms - if card fraud can be prevented effectively.
Social media is cheaper than that, on a per user basis (at least amortized across all users, very active users are a net cost, but also lead many others to participate). I don't know what the price would be, exactly. But let's estimate. Twitter has approximately 300 million users, and brings in around $2.5 billion to $3 billion a year (over the past several years) in revenue. So to get an ad free experience and, assuming they're at least breaking even, cover their costs would be about $10/user.
That isn't to say they'd drop paid content, but they could.