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Sure, well users started doing the "RT: ...." thing, which prompted Twitter to add the feature.

Yup, but I think even when users started doing that, they were inspired by Tumblr's reblog and similar mechanisms.

(Actually, I think Livejournal had this even earlier, though I couldn't swear to it)

Kinda weird that at this level it's still the same old game. One user makes the content (Tumblr), another copies and gets viral (Twitter).