You can't. Servies like these grow because of celebrities and their fans. Taxing the celebs will stop their own growth. Also, there are lots of would-be celebs who don't make a lot of money and are trying to use twitter to get the word out there.
> Charge third party developers for increased API capacity.
For the same reason as above, you can't. A lot of people come on facebook and stay there because of apps, charging devs for API access will kill innovation. Also, if something is popular and growing, taxing it will be bad for both the users and the dev if the dev can't pay. Also keep in mind that most popular apps may not have a monetization strategy yet.
> Charge users for special account abilities- like, perhaps, many of the features offered by tweet deck.
This is a good approach and is the LinkedIn model. I'm sure they're thinking of doing this with talk of 'lightweight analytics' etc so long as they don't make the paid users too powerful and disenfranchise existing users.