Can you use "say" to refer to things on the internet?

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:

He wrote that the word 'use' is often used wrongly.

not

He said that the word 'use' is often used wrongly. (Did he actually say it? Or did he post it?)

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:

• (of a text or a symbolic representation) convey specified information or instructions
• enable a listener or reader to learn or understand something by conveying or revealing (information or ideas)

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."