How can Twitter make money?

I was reading this blog post, click here, that describes Twitter and how various companies, retailers etc. use it to alert millions of people about special sales and promotions. One quote states that Dell alone generated $1 million in sales from twitter announcements. The odd thing is that Twitter is free for people and brands to use and cannot figure out a way to generate revenue. It’s another mobile / social networking / digital media company with a cool product and no revenue. The most interesting points was the discussion on technology that could help track in-store sales back to twitter in a more robust way to provide deeper analysis on how effective Twitter is on driving sales for retailers. Plus, RevTrax got a shout-out which was nice. Check out the blog posting if you get the chance. If you don’t even know what Twitter is, then feel free to pass on this.