Does a bar of soap ever get dirty? 

The short answer, while counterintuitive, is yes. A bar of soap can get dirty!  Many of us have walked up to a sink to wash, only to find a white bar of soap with brown streaks all over it, still covered in light film of the mud of the previous guy to use it.  It’s an easy fix.  You just pick it up and roll it over in you hands, under the water, and eventually the bar is clean again.

Being a typical (white) American, I don't usually use a washcloth to clean off in the shower. I just use my hands to wash my body off, and soap of course.  I do have washcloths, and use them, just not typically in the shower.  Don’t blame that on bad parenting. Mom used to give them to me when I was a kid, but I found them unnecessary. I was taught that you should wash from the top down. That way the dirt washed off over the still dirty parts of your body, instead of the part that you have already cleaned. As an adult, that logic rings true with most dirty items I’ve cleaned.
Over my years, I've also heard people say that when you're showering you should wash your butt last. Obviously, the logic would be because it's the dirtiest part of your body, and you certainly don't want to spread your ass germs all over the rest your body, right? Both theories seem to hold some validity.

The first theory makes the assumption that once it’s clean, it’s clean. Therefore, once you’ve washed you butt, you can go ahead and wash you legs without getting “butt germs” on them.  This might be where the washcloth comes into play, because I can definitely see the argument that once it has augured around your anus, it is compromised and should be sent to the laundry.  To continue to use it after washing your *ss, seems pretty gross.  And, if you are using your washcloth again the next day…  (skeeve shiver)

Now, considering with the second theory, if you washed you butt last yesterday, while your face, neck and ears are the first thing you’ll wash today. You’re gonna want to make sure you get that soap really clean when you get done using it the day before, otherwise you got leftover ass on your soap!
So, let’s break this down. It’s the day before. You just had at least one hand soaping up your stinky hole, it’s all rinsed off, and now you have your hands to consider.  Surely you're going to pick the soap up and wash them off (unless you’re one of those people who actually rubs the bar of soap all over your body, too). I mean, your butt is “clean” now, but clean enough to eat off of?  When you touch the soap again, aren’t you potentially introducing ass germs to the soap? Does that mean you should put the soap down, finish washing your hands, then pick the soap up and wash the soap? 
How long does it take washing the bar before it gets clean? Let's consider a bar of Ivory soap (which we all know is 99 and 44/100% pure). How many times do you have to roll that thing over in your hands before it's clean? If it’s got the previously mentioned dudes’ mud on it, maybe 3-5 flips, under the water.  What if it’s got invisible ass germs on it? Until I feel the bar getting smaller? We’re starting to border on compulsion here.

I’ve switched to a liquid soap…