Paid Signal Providers : Here people who weren't able to make money out of trading do this kind of scams. People are requested to subscribe their channels, so that you'll get daily trading signals. Further they do it as a paid service. Here if you're new to the cryptocurrency network, they easily scam you. Good is to do the best research of your own, because we've got lots and lots of legitimate platforms, YouTube channels are there using which you can know better about the market.
Those paid signal providers are indeed bunch of scammers that gives out random "signals". Most of the information they provide can be found easily on public. But I wouldn't suggest people to watch youtube to know better about the market. 99% of those videos are biased/paid.
Money Doubling : In recent days this is done through YouTube channels and telegram channels. 2X bots were developed and those were promoted. If you invest $100, you'll receive $200 through the bot. Person who promote this will talk in a very pleasant manner. Don't fall for the words. Stay cautious with such schemes.
You have to be really stupid to fall for a scam like this. I mean, those who get scammed, what were they thinking? Some random guy gonna make him rich? I don't feel bad for people that falls for scam like this.
MLM : This is a commonly known scam which is much seen on every form of businesses. It hasn't left cryptocurrency as well. Here based on the members added will be the commission and through this you can make big earning. One who is in the top order makes money scamming the down members. Even they'll say that you'll turn millionaire within few months. Most of the people who gets scammed on this schemes were the college students.
Those are not MLM. Those are ponzie schemes. As long as people keep on joining, they receive rewards and the scammers gain their trust. Once more people starts to join, the scammers make an exit, leaving everyone empty handed.