So where does that leave the US?
There's a small handful of carpooling apps operating stateside, including Coride and Zimride (which is actually a subsidiary of Enterprise Rent-A-Car). Have you heard of them? Yeah, no. That's the thing: none of them are popular, which suggests that Americans aren't really into hitching rides.
We have a unique driving culture here. We love to drive, and we like doing it in our own cars. There's also the issue of geography. In Western Europe, traveling by car between cities, or even countries for that matter, takes a matter of hours -- which isn't always the case in this big ol' nation of ours. BlaBlaCar's success here in the US of A hinges on it finding a niche of users on the East and West coasts, where drives between popular destinations don't take a full day and a half.