Okay, I'll bite on some of this.
marketing over Twitter
Compared to email marketing? For which I cannot enforce opt-in? Which deluges me in content that I don't necessarily have time to read? Shall I even bother comparing it to telemarketing -- is there any question what will win that comparison? Advantage: Twitter.
customer support over Twitter
I've never heard of one-to-one customer support over Twitter. Surely email is better, except when the spam filters intervene. For FAQs there are wikis and forums and good old web pages and new-fangled ticket tracking sites.
But what about generic, time-sensitive or high-priority hints, suitable for a sizeable fraction of the userbase? "Sorry for the outage; we are working on it." "Everyone should know that there's a security bug that has just been fixed." "Today 50 people have asked about Feature X. It's right there in the Bookmarks menu. Read the updated FAQ." These are the kinds of messages that aren't a good fit for bulk email. There's never really been a good venue for them before. [1] Twitter is going to spawn a lot more of them.
I'm not sure about payments over Twitter, but I know better than to laugh: I remember when PayPal was hilariously funny.
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[1] Okay, RSS feeds. But Twitter is, among other things, an RSS feed that non-technical humans can understand. And that is constrained to be blessedly short. And there are other semantic and social differences.
Comcast http://twitter.com/ComcastCares
We have been here before. I remember a company in 2001 that built some sort of business app that routed all its traffic over the public Gnutella network - just because. When I see people doing something over Twitter just because, that's overhyped.