Or you could text them directly. This scenario is only useful in the case where you have SMS but no internet, and your intended recipient has internet but no SMS.

EDIT: Also, as someone pointed out above, Twitter already has an SMS interface.

They're just a terrible example cause they're the one service you might already have access to that way. Twitter's only got 232 million users so anything that hinges on them is immediately not as useful as something that makes more of the internet - fb, email etc - available to more people, especially from someone else's device.