I would consider a carshare if:
I drove to work (which I don't currently) at an out of town location (hospital, business park, industrial estate etc), and a colleague was going the same way, and it was mutually convenient; or
I was going to e.g. a large music festival and others were going the same way (sometimes the festival organisers will link their website to a car share portal). This has worked well in the past
But I wouldn't bother looking for one-off car journeys even for long distances - the chance of finding someone who wants to do a similar journey at a similar time without significant diversions is small, and on corridors where there *are* lots of people going the same way, there is usually a railway!
If I was a non-driver and desperate to save costs, I would have thought traditional hitch hiking at a motorway slip road would be more likely to find a willing driver than a rather esoteric website.