What should I say?
Say whatever the writer's article makes you feel - provided it is not threatening or slandering in nature. There is no exact way to comment, as long as you are not violating the TOU (or TOS) of the site or breaking the law. However, it is nice to leave a comment that signifies to the writer how you felt about the piece or the subject matter.
Think about the author and readers.
Some may be offended by certain comments or those that read "PV love" or other similar statements. Even if it may not be your intention, some may feel you are only reading the article for reciprocal views. Also, outside readers may not understand the meaning behind such comments. There is nothing wrong with helping out fellow writers. But, putting some thought into your words may be appreciated even more.
Just be you.
This is something I am very adamant about in everything I do online and off. Be yourself. Allow your personality to shine through in your comments. There's no need to be reserved if that's not you. If you're reading an article, but don't know much about the topic other than what your reading, don't be afraid to admit it. The author will be glad to know you appreciate the lesson.
Share personal experiences, but in moderation.
If you feel like it, go ahead and share your life in comments. That can be a great addition to an article. But, think before you speak - or rather, type. There are some things that you just don't tell everyone. If the writer is a friend of yours, you might send a private message instead. A good rule of thumb: if you are not alright with everyone you know reading it, don't say it. You never know who might come across what on the internet. Also potential employers or clients might Google your name to see what you're up to.
Don't have expectations.
If you leave a comment on someone else's article, they may choose to reciprocate. However, don't assume they will or expect them to. Also, remember that just because someone does not leave a comment does not mean they did not read your work. Speaking for myself, because my days are busy, I do not leave a comment on many of the articles I read.
Be considerate of other people's time and choices.
Ever received a message from someone asking you to read an article you've already read because you hadn't left a comment? Please don't send such messages. If someone wants to read your article, they will - and they may not always leave a comment. No one should feel obligated to leave a comment on every article they read - or to read every article from their favorites or followers. When would there be time to write or spend time with our families? Instead, simply leave comments when you are able and appreciate the comments you receive as well. Bonus: Less stress from worrying about whether you or someone else has commented or not.
*I originally published this via Yahoo Contributor Network