Twitter can monetize anytime. They have a huge number of options, because the size of the userbase makes it possible. Even silly things will work.
For example, let's say it costs $1 to write a twit that snoop dogg will actually see. A percentage of his fans will do so. Spread that across the thousands of people who have a huge number of fans, and you already have a revenue stream. Combine with a number of other things, and it here will all become quite profitable.
This is the same faulty logic that's killed hundreds of web startups during both the web booms.
Massive user base does not autotmatically translate into profit. In fact, what a massive user base does do is to create so much cost in running the service that it's hard to find a way to make a profit.
Twitter's already $55 million dollars down which shows just how much it costs to run the service. So even if every one of Snoop Dogg's 25,000 followers were willing to pay a dollar (unlikely) and Twitter got all the profit from it the money wouldn't make a dent in the cost of running such a massive operation.
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.
Why should SMS cost twitter anything, when cellular providers make so much money charging consumers for SMS because of twitter? They are a large enough source of astronomically profitable SMS messages sent by cell phone users that the cell phone companies should be paying twitter a piece of the SMS action, and may already be?
Why hasn't that business model worked with any other form of communication? Lack of verifiability and the fact that anyone someone would pay money to talk to probably has better things to do than getting paid $3 / hour reading tweets.
For example, let's say it costs $1 to write a twit that snoop dogg will actually see. A percentage of his fans will do so. Spread that across the thousands of people who have a huge number of fans, and you already have a revenue stream. Combine with a number of other things, and it here will all become quite profitable.