OTOH, I frequently scream with laughter, or silently agree with an opinion, without necessarily posting or PMing in reply. Silence, in other words, can't always be taken as withering condemnation!

I agree with maverick on this one. Sometimes a post will be quite funny, profound or interesting but no response will be made since none is needed. The particular avenue might also just have petered out. Plus if we all start posting just to say, "that was interesting, or smart, or funny" the threads would become way too unwieldy.

As to ignoring someone when in disagreement, well, this is the written word, how can a person tell whether you are voicing silent disagreement or silent approval. When a person is convinced he is right, he will believe everyone agrees with him because nobody spoke up.