Let's say you have your Private. You want to go flying and don't want to go alone. You call your friend up and see if he wants to come flying for a pro rata share. He agrees.
Does this count as "common purpose?"
No. The FAA Chief Counsel has repeatedly stated that "common purpose" means you and your passengers both have reason (not necessarily the same reason, but you each do have a reason) to be going to whatever destination on that particular day. Going for a joy ride isn't going to be acceptable, especially if you propose the idea and your passenger accepts -- that sounds too much like "holding out" an offer of airplane rides for pay.
And let's say the answer is "no, because you might of just been doing it to build flight time so you wouldn't share a common purpose." Ok. How does the FAA know/prove that your motive wasn't simply to go on a joyride with your friend who also wants to go on a joyride?
The FAA isn't out there trolling for violations of this rule. The FAA knows about such violations because someone complained to them about it, and in a situation such as you describe, that someone is usually the passenger involved. In that case, the passenger's testimony will be quite enough to convince the judge hearing the case that it is more likely than not that you did what the passenger says you did. If you do this often enough and get a local reputation of giving "joyrides for pay", one of the local FBO's may get wind of it and make the complaint since that would sound like you're horning in on their sightseeing ride business. Again, if the FAA gets that complaint and investigates, somebody is going to say, "Oh, yeah, he took me up flying and asked me to pay for the ride."
Also, how does the FAA enforce pro rata payments? Can they do some kind of audit on your expenses? Is it purely honor system along with spot enforcement (i.e. FBO reports you)?
The FAA can indeed subpoena records as needed, but even without that, they would have no trouble presenting sufficient evidence of the approximate direct cost of the flight to convince the judge hearing the case that $100 each wasn't an appropriate pro
rata share for three passengers getting 1-hour ride in a 172.
All that said, it is generally an honor system, but if you get a reputation for giving paid rides, eventually someone is going to complain. Likewise, if you are seen taking people places that folks know you have no reason to go other than to fly them, again, someone will complain. If the complaint happens, and you really were violating the rules, the FAA is not likely to have much trouble sustaining an enforcement action against you under the relaxed "more likely than not" standard of proof in these cases (as opposed to the "beyond reasonable doubt" standard in criminal actions).
So if you're going to do this, don't post it on the internet, don't create any records of the flight, don't take people you can't trust to keep their mouths shut about it, and don't do anything on the flight to attract the FAA's attention (like a crash or flight violation).
