Digg, Friendster, MySpace just off the top of my head.

It's a bit of a counterfactual, because startups with large traction having difficulty monetizing don't typically go bankrupt, but sell. But that doesn't prove they wouldn't have been able to monetize at some point, and leaves us guessing.

One example is YouTube; they lost huge buckets of money in their entire independent existence, until Google's acquisition completely changed the economics of their bandwidth costs. Would they have been able to become profitable if they stayed independent? Maybe!

Another one (perhaps clearer) is Broadcast.com. Never turned a profit in their independent existence; never turned a profit for Yahoo after acquisition, either.

Digg, Myspace, BitTorrent, etc... at varying degrees were all able to monetize users and made millions upon millions of dollars while doing so. In fact, this answer from the founder of BitTorrent is to my point.

http://www.quora.com/How-profitable-is-BitTorrent-Inc

"Our main source of revenue is off the toolbars we push out, which isn't a great source of monetization, but it's something. You can do the math on how many installs we have and what the monetization of toolbars is, and subtract out the costs of having 50 employees, and you'll get a reasonable ballpark of how profitable we are, although the short answer is very. Even lousy monetization works well when you have more users than Twitter."

ChatRoulette, Napster, BitTorrent, PointCast, RockYou, etc etc

I'll +1 this if I can add the stipulation that it's a US-focused b2c play, mostly because I don't know the international space well.