But doesn't that get rid of the social aspect of "Social Media"? People like being able to comment on things - but why should they have to pay to do so? Just because someone isn't willing to pay doesn't mean that what they've got to say isn't worthwhile, and vice versa. I think a pay-to-post/free to consume setup would result in a spammy, advert ridden service, that would quickly die.
Agreed. What I see most people using Twitter for is effectively advertising anyway, so let those people pay to keep the service running since they're clearly getting marketing value from it.
You should let people follow anything they want for free, but if they want to turn on the ability to publish content into the service, then they pay