Were I a Twitter client developer, I would get in touch with other client developers and start talking about a way to do what Twitter does but that doesn’t require Twitter itself (or any specific company or service).

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Under the hood, following somebody is really just subscribing to a feed of their statuses. Posting is really just updating a feed of your own statuses.¹

Brent Simmons writes about Twitter’s changes to its API rules and threats to third party developer. He brings up an interesting idea, that developers could create their own decentralized Twitter-like experience.²

Sure it would likely be just for geeks, but I think third party clients are almost exclusively used by more or less geeky people and Twitter itself was once used solely by geeks.