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Via @fredbenenson and @gasca: this is (now, didn't used to be) one of the default ways Twitter provides new usernames!
I presume it means that people who don't bother changing the default aren't very invested in tweeting well, thus my pretty negative impression.
Excuse me, allow me to correct that: _on average_ this may be true. It is _not_ true of many individuals (which explains something that was perplexing me - thoughtful comments from such accounts).
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