This sounds right to me: most of the comments I've written, on most sites, haven't been useful to me socially. The people I was talking to were strangers and remained strangers. But sometimes the act of commenting helped me understand something; sometimes my comments helped other people understand something; and sometimes, very often, reading other people's comments helped me understand something. So it's not valuable for socializing, but it is valuable for other things.
That said, there's another problem with comment culture that seems worth mentioning. I seem to have gotten good at expressing thoughts that fit in a comment. That gives me a false sense of competence; but when I need to write something longer, like a blog post several pages long, the structure just completely falls apart. And writing a book I can't even imagine. It seems writing at different lengths is essentially different skills, which need to be practiced separately. And if that's the case, then as Annie Dillard says, why not just write something long to begin with? I'm actually thinking of that now.
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