Sorry for the delayed reply...

I don't know what Twitter is like, but the function on Facebook that I prefer to use (private messages) is almost like email and seems to be replacing email among much of my social circle. I will preferentially send my friends FB messages instead of emails, since I usually get a reply faster.

Writing on someone's wall is public, and might result in a slower reply because it seems less urgent. But it's still directed at a particular person, and it would be considered rude not to reply at all. But when I post an article or link, the reply I often get is "thanks, looks neat, I'll read that later."