Google adsense is the only one I will mention...

It's sorta like a favorite fishing hole, if you tell all your friends about it, suddenly one day the fish are gone and the place is so packed you can't even find a place to sit down anymore.

So I am leery to reveal any unknown revenue streams for the simple fact that those of us been around for a while have learned the cold, hard lessons that jealousy and competition bring along. Not to say everyone is jealous but it appears folks who are unwilling to reveal their main source of income likely has to do so for similar reasons, we done found our secret corner and we don't want the whole world interrupting our little revenue stream... Because once that happens, we get to start all over again.

Another way to say it is it has to do with supply and demand: Find a place where your supply satisfies the demand (or just under satisfaction but not too far), and you done found your corner. Invite everyone else thou, and you may find the demand no longer satisfies your supply, which in turn reduces your revenue.