My defense when I was younger was simply that I didn't even intend to type proper sentences for instant messaging -- I viewed it as a textual representation of speaking[1], not short-form prose.

(These days I aim for proper sentences also in instant messaging, acknowledging the asynchronous nature of it. But if I happen to catch my conversation partner active, I might switch to the more informal, "verbal" style.)

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[1]: You may think you speak in proper sentences, but try to transcribe an audio recording literally some day. You might be surprised!