Well, I guess you can say LinkedIn is not a success (and with a PE of almost 1000 I like to be indignant-on-the-Internet, too). But if you ask most startup founders if they'd feel getting to LinkedIn position is a success, I think you'd get a resounding, unanimous, yes.
Ditto for reddit. Not sure how well the RapGenius people have made out but it seems they're doing well.
Remember, being a viable business is different than being successful. Twitter hasn't turned a profit, but the people that started it made off quite well. Splunk still loses cash left-and-right, yet is considered solid.
If I came up with a money-losing idea that got millions of users, never made a dime, and I sold it for even $10M, I'd be very happy with my "failure".