ST
I think that regulation puts it quite well. If a pilot is doing a specific flight anyway, puts it on a cost share site and, as a result, gets a rider prepared to share the cost, that's OK. If they simply advertise along the lines: "I'll take anyone anywhere they want to go if they pay" that clearly isn't.
One grey area I noticed with Skyuber was that potential riders can post flights they are interested in taking. In most cases, I believe that to be innocent enough: a pilot wanting to take that up would still have to post the flight and wait for the rider to join it; but still I can see how that feature could be abused.