Twitter has one thing going against it that is virtually unique: costs are super-scalar to the number of users, whereas in almost any other content-based website they are logarithmic.
The reason is SMS charges. Every user who signs up for SMS costs Twitter real cash money to service at the margin, and their cost increases as the size of that person's personal network increases. If Joe SmartPhone signs up to follow me on Twitter, every one of my status updates starts costing them money, too, despite the fact that I only use their web interface.
There is literally no other web service I use that costs my provider a per-use fee, except my brokerage account -- and they, quite sensibly, pass that per-use fee and then some straight to me.
The reason is SMS charges. Every user who signs up for SMS costs Twitter real cash money to service at the margin, and their cost increases as the size of that person's personal network increases. If Joe SmartPhone signs up to follow me on Twitter, every one of my status updates starts costing them money, too, despite the fact that I only use their web interface.
There is literally no other web service I use that costs my provider a per-use fee, except my brokerage account -- and they, quite sensibly, pass that per-use fee and then some straight to me.