Contacting authors at conference after refereeing their work
This is just my opinion; in some conferences and journals, it is specifically mentioned that the review process is blind and the authors should not know who the reviewers are.

If I was the author, I would be more comfortable with your opinion about my work, without mentioning that you have reviewed the paper.

You can simply say: "Look, I have read your paper and listened to your talk, some might say ..., but I believe .... You'd better work on ... etc"

You don't need to mention that you reviewed it at all (although they can pick it up quickly). If you explicitly mentioned it, you can put the authors in an awkward situation.