I "acknowledge" motorists in situations where I want to express some intention or exert some control, like when signaling. Also at times with friendly waves as motorists pass safely. Especially if they had to wait behind me for a few seconds.
Acknowledge every passing motorist? No, that would not be practical, nor necessary.
Motorists acknowledge me as they approach from behind by moving left or slowing down. This requires I ride with a mirror and keep tabs on them, which I find very quick and easy to do. This also assumes a lane and shoulder situation that makes it unsafe or impossible to be completely "out of the way". (which is the majority of roads, at least around here) If I'm riding in a wide shoulder, or wide lane, there is usually no acknowledgment either way.