Given that the major appeal of Twitter is the ease with which you can reach effectively unlimited numbers of people, I doubt that would be a very popular move.

Given that the major appeal of Twitter is the ease with which you can reach effectively unlimited numbers of people, I doubt that would be a very popular move. It might be more feasible to charge people for following other people. Or just offer a straightforward ad-free experience (maybe with some extra features) for a set monthly fee. Which is what I've wanted all along, but has the disadvantage of fracturing the userbase you can sell ads to and making Twitter a less valuable platform for advertisers, so they'd need a decent conversion rate to offset that. I now absolutely hate this thing where the internet is made of free stuff and people won't pay for it. I know a load of people who practically live on Facebook but the idea of paying for it horrifies them. Do not understand that.