There's upsides for Twitter: when 90% of my friends get the $100 plan there's a huge incentive for me to do the same. When all of my friends keep the free plan then there's no incentive for a loser like me to pollute the higher Twitter echelons.
Might be some tricky cases where I follow a celebrity who has the $1000 plan.
I know these ideas sound ridiculous. However considering the mainstream nature of these services and peoples' insatiable appetite for competition and one-upping each other it could work out. The prices for each tier would probably need some iteration to get closer to some global maximum.
Also I wonder if this could work for Facebook?