Twitter if course is by an large a free service, but it’s not really a service that has no business model. Currently Twitter has the ability to let businesses pay for trending tweets (or whatever they actually call it). Now I don’t think that this really a model that is supporting them fulltime, I really wish Twitter would charge a bit for their Apps. I would pay a few dollars for the iOS Apps and the Mac App.
I don’t think that Twitter does this because it feels it can make more money by allowing trending tweets and injecting things in your timeline. That is certainly their decision to make, but it does make the users the product and the advertizers the customers (much like Google’s business model). I wonder if they will add a bunch of ‘crap’ to users timelines and then drive away users? I certainly don’t want to see any trending topics or promoted tweets in my timeline.