Wattpad is another social reading writing website. It lets you highlight a quote then make a public comment. This is good for telling the writer about typing mistakes.

I only make conservative suggestions such as spelling correction, one word grammar correction, adding or subtracting one word.

To make more complicated edits I would need to talk to the writer to find out how they want the phrase or paragraph.

Giving edits in comments has the advantage that you can show multiple alternatives and explain why you think there should be an edit. Also, a comment is compact.

It takes time to look through a long issue for sparse suggested edit. But it's hard to make a quote in a comment. I think copying should be allowed if the story uses a liberal licence like creative commons or public domain.