The other day, a friend of mine got stood up. When she confronted her date, he protested: he had tweeted that he'd be running late. An extreme example, but part of something that must be addressed!
Twitter is still, relatively speaking, in its infancy. (Or, like dogs, does social media age at some accelerated rate?) We are still figuring out its relationship to our lives and the eventual role it will play. It's fun, it can be informative and entertaining. Some celebrities have revealed themselves to be masters of the art-form, while for others it's an instant source of regular TMI. And, yes, dealing with like-minded friends and strangers is one of its primary pleasures - threads of quick-fire jokes and real-time reactions are a great way to interact. However: 140 characters on a public space is no substitute for direct communication.
If we were to rank communication by directness, the list might go: Face-to-Face; telephone or skype; text; IM; email; Facebook wall; Facebook message; Twitter "direct" message; Tweet. (Letter is of course recused, since it compensates for delay with increased personality.) Normally, one only resorts to Facebook messages or Twitter direct when a more exalted avenue is not available: if you have someone's actual email address, you'll probably save yourself both the trouble of - again! - being reminded that your message could not be delivered because you accidentally responded to the server and you'll need to login and communicate directly via Facebook.
"Tweeting" communications are different. Generally, they're between real-life acquaintances who, presumably, have your actual contact information. But they're inefficient: you cannot assume someone will receive the message. What's more, even if someone is sure to receive said messages, they're oddly public, meaningless to everyone else, and at worst seem designed to suggest a deliberate exclusivity.
All this said, I've been guilty of it. Sometimes when someone's right there, and you want to say something that's not really worth a whole email, it feels weird to inundate them with multiple media at once over some small matter of will-you-be-at-that-show, almost like a petty conjuring trick. And one could argue that there's a pleasing old-fashionedness to Tweeting - yeah, maybe they won't get it, but the little neighbor boy who would have delivered a message for a penny in ye olden days might have been distracted by a lemon drop or something, too.
In any event, no one's suggesting this is an issue of national importance, although surely students of media could make a little hay. Rather, it's a petty, passing irritation, of no more moment or duration than the 140 characters that are its building blocks. More to the point, my friend regarded the whole thing as a red flag and declined a make-up date with the gung-ho Twit.