Pete Cashmore has a blog post that covers this well with a screenshot so I won't repeat the details here. 

This brings up the question, where does the line get drawn with others copying? Being copied is supposed to be the ultimate form of flattery but.. facebook put twitter-like functionality, now bebo, next My Space, Flickr and everyone else (guessing here!). The good thing for Twitter is that they have momentum and a community starting around their service. They also have the brand name so I think it would be difficult for a new standalone Twitter to be successful. A Yahoo, Google, Facebook, My Space could come out on top though, the ones with the existing users could. We will see.

David Chartier has a few ideas on how Twitter will make money as a company and he also briefly chats to Evan Williams from Obvious co. / Twitter Inc. about it. The quick overview of the ideas are:

1. Twitter Pro - Charge for some upgraded services
2. Merchandise - Sell stuff
3. Twitter for Business
4. Ad supported Forums
5. Advertising
6. Sponsorships

Also this comes to mind. Twitter must be giving the phone companies huge revenue in SMS charges. Ev has said in an interview (can't remember where) that they are actually paying these bills at a discounted rate. These are only the updates they send out. How many more people are sending updates TO twitter and generating revenue for the phone companies. Maybe there is a way to share in some of these profits. Difficult but possible if they can prove the volume of these messages.

The last way off the top of my head is data mining. They have all that data, they must be able to use that somehow. Someone must want it and find it useful if it is sliced and diced and presented nicely.

With the exponential growth of twitter in just a few months, there has been some talk that Twitter and Ruby on Rails may not be able to scale. There are some posts today that say that is not the case. Anyway, I am sure that the guys at Obvious and Twitter will FIND a way to make it scale. Every new user and page-view on their hockey-stick-like usage graphs is like cash in the bank. Ok, more like a loan or a promissary note maybe. The point is that they will find the technical solution and the cash to do it, whatever it takes. More users and popularity has got to be adding to their valuation by the month, week, day and even hour. The 64,000 dollar question remains however. Will they sell out our keep growing it into a revenue producing business?

Oh, and if you are interested in those posts, they are here and a more technically minded one here .