Twitter knows when you sleep. In Istanbul, for example, most Twitter users don't sleep too much in August, probably because they're strolling down Istiklal boulevard. In Sao Paulo, Brazil, tweets tend to die off in the afternoon – right when folks are taking a siesta. New Yorkers, meanwhile, never stop tweeting – even late at night.
Why is this information important? For advertising, of course! Advertisers want to target their customers via Twitter when they're actually using the service, and thus all this information about user activity is incredibly lucrative to marketing teams. An advertiser in Brasil, for example, may want to target Sao Paulo residents after they've woken up from their naps.
Twitter also tracks their trending stories and has found that 17% of all top queries change every hour or so. The hivemind is constantly switching topics, and the key for advertisers is to maintain relevence. In the wake of Steve Jobs' death, for example, the query "steve jobs" spiked from a "negligable fraction" of the query volume to 15% of the query stream – or almost 1 in every six tweets!
The data that social sites like Twitter is sitting on is worth big money. And that's how all these sites work, Facebook included. After all, if the service is free, you are the product they're selling.