In France this sector is working very well (covoiturage.fr), has been for several years, to the point that friends consider checking out rideshare site like that and price-checking with trains and planes.
I have a suspicion that the fact that a lot of rides are either Paris-X or X-Paris probably helps (a lot of parisian students end up getting schools outside and go home during vacation/weekends). And students here don't always have cars.