I leave only positive comments on fics I liked (even if I did have criticism as well) and don't comment at all on fics I didn't like. That's pretty much how I leave fb at other times as well, because I don't really want to deal with angry authors (even if they say they want concrit in their headers, I won't say anything unless I really know for sure they mean it). I write reviews of everything I read on my journal anyway, so I can talk about any criticism I had there. I do write reviews of gift fics as well, but as with all my reviews I am careful to distinguish between bad writing and things that are just not my thing, but which others might enjoy. But the reviews are mainly for me to write my thoughts on what I read, which is why they're on my journal.