I posted an article on/in/at EnglishForward.com.

I posted an article on/in/at General English Grammar Questions of Englishforum.com.

I'd use 'on' in both. Is either in or at also acceptable?
Can I leave an article out if they know what I'm talking about?

Things get a little hazy one you enter the www.

It's easy to visualize posting an article ON a corkboard or bulletin board that is physically on a wall. You can also think of a forum as a frequently updated bulletin board that is physically on a wall.

But when it comes to the www, the idea "World Wide Web" is something IN which things are contained. We say "caught IN a web" -- not ON.

So with that logic, I would tend to use IN (to answer your first question):

I posted an article IN EnglishForward.com.

I posted an article IN THE General English Grammar Questions SECTION of EnglishForward.com.

But I'd understand it just the same if people would use ON. So I think ON is acceptable. AT is my last choice.

To address your second question, your sentence would be incomplete if you don't mention what you posted (or even use the word "it": I posted it...). BUT if your friends already know which section of the website you are talking about, then you can get away with dropping "an/the article."

"What do you mean you don't see it? I posted three minutes ago!"

**But I'm still tempted to insert "it" after the word "posted."
I will also use IN.Emotion: smile
Your comment really helped me understand this issue. Thank you very much, Arvsworld.

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No problem, LiJ.

I have a question:

Please which is correct;

I posted on the group page or I posted in the group page