When referring to something you read on the internet, can you use "say?" For example, can you say, "he said it on a forum" when speaking about a message someone posted. Or does say only refer to spoken words?
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This is commonly done. However, "Say" could possibly cause confusion, which you have noted. There could be some confusion over whether the user actually said that, or just posted it.
I think the best way to avoid this, would be to use "wrote it". For example:
Of course, usually this can be determined by the context (e.g. Language hacker posted on a forum last week. He said that...).
But to be on the safe side, "wrote" would be better.
Two of the meanings reported by the NOAD for say are:
In the second case, "I don't want to say too much" can used for who is listening or who is reading. Say can also be used figuratively as in, "the movie's title says it all."
"Say" can indeed be used to refer to expressing ideas in written form. Compare "What does his letter say?" and "The sign says 'STOP'".