A good comment but I think you have it backward. When NBC puts "tweet this story" in the HTML frame that isn't advertising for twitter per se, that is NBC hoping that you will give NBC free advertising by spreading awareness of a story around the web. When the Tahoe ski resorts say "Take Interstate 80 to Truckee" they aren't advertising I80, but they are causing a lot of traffic to be generated, and that traffic is going to cause road damage that needs repair.
This is what I mean by "figuring out their (Twitter's) value."
Imagine that their API is really a bunch of method calls into the Twitter 'object'. They could, as an example, make their simple API 'free' and as you tried to do more complex things charge for those things. Presumably they would structure it so that folks could make 'useful' twitter clients for free, but they would have to pay some price for making more full featured clients.
Of course the challenge there is that you can't really charge for features that can be built out of the free protocol stubs, because people will just scrape those free ones and get around your code.
This is what I mean by "figuring out their (Twitter's) value."
Imagine that their API is really a bunch of method calls into the Twitter 'object'. They could, as an example, make their simple API 'free' and as you tried to do more complex things charge for those things. Presumably they would structure it so that folks could make 'useful' twitter clients for free, but they would have to pay some price for making more full featured clients.
Of course the challenge there is that you can't really charge for features that can be built out of the free protocol stubs, because people will just scrape those free ones and get around your code.