I did it with a little success back in forex days, long long ago before Bitcoin when eToro was starting up. Pips and all that I forgot the names but you could give away $5 free per signup and get $5 regardless KYC and regardless amount traded. Those were gold old days 
I think the problem started with bots and multi accounts. Ruined it for us who actually did great promotion
Now, not worth unless you invest serious money into marketing. So if someone in crypto says easy money, I say scam :p
Unfortunately that is how the abuse works, when there is money to be made anywhere in the world, people jump in and try to figure out a way to make as much money as possible and leave nothing to anyone else.I think the problem started with bots and multi accounts. Ruined it for us who actually did great promotion
Now, not worth unless you invest serious money into marketing. So if someone in crypto says easy money, I say scam :p
Even if there was an option of them making 10 and everyone else making 100 or them making 20 and nobody else ever makes profit, like not all, just a bit more, they would still pick that. I am not that kind of person, if I find a way to make money, I just try to make a steady income as long as possible without disturbing it, that way I could make money for longer period of time. So far I hit one of those unfortunately lol, still make 20$ per month, been 7 years, not a big deal, but definitely a good feeling without a doubt.