Perhaps they are afraid that many criminals will use them as tools for fraud or maybe in the future they will release their own crypto

Partially it's for that, but I think its because government regulations, if they promote scams and ponzi schemes they will be responsible for that. As I see they ban didn't pass well, many people still advertise crypto on this two social networks.
I believe they will not be able to ban crypto advertising entirely. They are shutting down crypto accounts, but 3 new appears instantly, this crypto fever can't be stopped. We will see what will happen on the future and how will they deal with this problem, the thing is they need to work under the laws of the country where they are registered, their decisions will depends mostly from that.

Facebook and google are in the view that cryptocurrency is a scam and think by banning the advertisements, they will protect their users but they have not realised that the cryptocurrency industry has the potentials to help these users.

I think,the one reason is,cryptocurrencies mostly the bitcoin,are very popular in many countries around the world. Banning of these advertisements,will avoid the possible clogging on their network operations.

It is difficult to distinguish between fraudulent advertisements and non-fraudulent ads.
So I wonder if they announced a ban.

They do simply don't want to harm people financially since bitcoin could definitely do such right. Also, may be they do have the same reason why the government bans crypto on their society. Else is that they are creating new virtual money on their own beside crypto and they don't want to brainwash their future user that is why they ban it as early as possible they could. Banning it is a really tough decision since advertising could help the market grow up once more.

There are many theories why .My of course that they want to capture this market and control it.Or the government is pressing very hard, because you know the crypt will humiliate the dollar and lower it as no one ever .But there are many of us,and we are not so easy to break, we will find a way out always !

Hi, I have two versions of what's happening:
1- All the same, if you think about the idea of ​​all social networks is a free communication. But hunters for rewards in the crypto world using the requirements of the bounty campaigns are very much abused (as much as the number of reposts) and this very litter the main threads in the social networks.
2 Which seems more plausible to me is the global world secret of the financial world. These financial kings see that with the departure of finance in the crypto currency they began to lose their profits and declared a cold war against bitcoin and the rest of the tokens.

That tells simply that it is not like adverts are banned totally, but as long as you have an ICO, you can tender documents to show its legitimacy, then you are good to go and this is the reason why you see some adverts of cryptocurrency still displaying. The good thing is that, at least, if you see an advert now, you sure would still be relaxed that it is verified and you can at least be compelled to want to dig further on the project if it is worth investing or not which for me, is a good thing.