Here's another good link:
Reigel Law Firm, Ltd., an Aviation Law Firm
Even if you 100% own the aircraft and pay 100% of the costs, the FAA can find a way if they really want to. There was a case (I saw it one of the mags, don't remember the reference) along the lines... my best customer comes to me and says "I really need to get to Vegas tonight and the last couple of flights are full. Any chance you could take me there?" So I do, and don't charge him a nickel. The FAA can maintain that the payment was in kind, i.e. the promise of future business.
The real killer, imo, is the risk of being sued. An example... a few years ago I offered to fly a good friend's daughter to her summer camp, 150 miles away. It would have been a nice day out for me and her parents. Then her friend wanted to come along too - someone unknown to me, the parents also. That was the end of that. The risk of being sued after an accident, or even nothing at all - the poor darling was traumatised by a go-around, or by a normal takeoff, and the next five years spent battling lawyers. No thanks.