500 million tweets a day means that users are getting some kind of positive reinforcement for posting, and that it's more than celebrities.
How many are spam?
How many are shady i.e. pay folk in the Philippines or Pakistan to send out tweets all day, to boost the numbers?
That's only 5,700 tweets a second.
An average IRC network for example does more than that.
Many users are following so many accounts that they can only read only a small subset of their timeline. I wonder if Twitter's apps and services also have to evolve substantially like Facebook's did to include more elaborate filtering and ranking options.