I'm going to answer because it seems no one else bothered. You are being downvoted because calling Blablacar "just an idea, no execution" is like telling Dropb

I'm going to answer because it seems no one else bothered. You are being downvoted because calling Blablacar "just an idea, no execution" is like telling Dropbox is "just an idea, no execution". Blablacar have executed very well, with a customer acquisition phase (when it was free), then stepped up the monetization (gradully stepping up fees, while keeping a healthy customer base). They also began expanding out of their home country (France), which is no small feat considering cultural differences. On the profitability part, what makes you say they are not profitable already? I can't really extrapolate numbers, but a good customer base + recurrent fee (for each travel) generally are good signs for a business. I would bet the raised money is to fund expansion in other countries (I am pretty sure at least one of the previous round was for that).