What this is about is that the FARs prohibit private pilots from flying passengers "for hire" but allow them to "share expenses." There are a number of tests to determine whether a private pilot who takes money from passengers is flying "for hire" or merely "sharing expenses" and one of them is the so-called "holding-out" rule: if you advertise your willingness to take passengers, then you are presumed to be flying for hire and you need (at least) a commercial license. Advertising on Flightshare is considered "holding out" by the FAA.
(I'm a private pilot.)
They are not licensed as airlines, so selling tickets at a profit is a definite no-no. But they can take their friends along for the ride and split the cost of fuel.
A number of apps/websites have popped up that blur the line between "operating an illegal airline" and "doing the equivalent of a carpool in a Cessna." That's the gray area that needs to be fleshed out.