There is a difference between passion and going-broke/burning out-because-the-site is exploding in popularity. The first can be fun, the second is work, and much of that work is probably not fun (sharding etc.) and not the creator's real passion.

At that point, the passion has to become a business because, well, that is where the effort is going to have to go. Maintaining it as a passion project is not longer sustainable and you have to choose.