I lived in Germany from 2009 - 2010 and ridesharing is absolutely huge. I haven't heard of anything happening. I probably used the ridesharing website from 2 to 20x per month (didn't have a car but traveled a lot). This is the one I used: http://mfg24.spiegel.de (the "spiegel" part is actually a major German publication but the rideshare itself is powered by another company). I'm sure chrome will translate it for all you non-germans ;) It's been around for 10+ years
I think it's been around even before the 'net, which is why several sites basically have the same name ("Mitfahrzentrale"). Student organisations and similar groups used to do it for their members or event participants.
I'd say that hitch-hiking isn't that more common than in other countries, but sharing a car for a long trip is. (And yes, dear Americans, I know, a few hundred miles really isn't long…)
I'm sure this service would also be an awesome way to make friends. Everyone's a stranger before you get to know them. So maybe it's worth meeting up before getting into a cross-country car ride with them.
It would become known if anything like that happened methinks. Most likely it would be all over the news as a "Don't meet strangers from the internets! The internets are dangerous omg!"