CL Rideshare is funny because like you say, it works for 18-50 year old males that are willing to research the "few simple rules" perfectly fine, so about half a million people. There's no way that half a million people, out of 330 million Americans, is the full market of people that would be willing to do a rideshare. I think there's a case to be made that Craigslist is keeping that number from growing by not improving the level of trust they add, and have made it difficult for others to innovate in the space by (justifiably or not) keeping it a walled garden.