As a private pilot the legal issues worry me. The pilot training, aircraft maintenance, and operating requirements are very different for different types operations. The "sharing costs" is based on the concept that you can fly your friends to Las Vegas and split the costs. It is assumed that you have reasonable informed your friends of the risks. If you are taking other "passengers" for some form of compensation, have they *really* been informed of the risks - which are dramatically higher for private flights than for air carriers.
If there is an aircraft malfunction and someone is injured, what are the insurance / lawsuit issues? what happens if a passenger damages your airplane - stepping in the wrong place, can do thousands of dollars of damage to some planes. What if you can't reach the intended destination due to weather - does the passenger get a refund? What if you are delayed? It is legal for private flights to operate under weather conditions that are not legal for commercial flights -what happens here? Fuel is less than 1/2 the total operating costs for my plane - do I get to split all costs, or just fuel?
We are also talking a lot of money here. A Bonanza or Cirrus total operating cost is probably ~$200/hour, so a "quick flight" from San Francisco to Las Vegas is $1000 round trip, close to 2X that in my Baron. Non-pilot passengers may expect a level of service and performance that just isn't reasonable for small planes.
Its a nice idea, and I'd love to participate, but there are too many possible problems.