Though the idea seems new, flight sharing has been around for decades–even dating back to the 1960s, when pilots posted their trips on bulletin boards at local airports. Today, many private pilots split fuel costs when taking trips with family, friends, and co-workers. This type of flight sharing is referred to as “common purpose.” These practices clearly were never meant for private pilots to “hold out” to the public for hire and compensation.
Source: aopaGroups urge caution on flight sharing