Checkup alladvantage - they paid people to surf. Had millions of users, but ultimately failed because their "business model" was idiotic.
Getting millions of users is pretty easy if you pay them to be a user. Someday though, it's only worthwhile if you can build a sustainable business which at least doesn't lose money hand over fist.
If Reddit hadn't got bought and supported by other profitable businesses, I doubt it would have survived.