Cryptocurrencies right now have a trade off between scalability and decentralization. Bitcoin is theoretically decentralized even though the reality is more complex. But theoretically anyone with a computer can randomly decide to mine the coin and participate in the network. This whole process is made possible by Proof of Work, which was the key brilliant idea that made bitcoin secure and possible. The problem right now is this algorithm doesn’t allow very many transactions per second.
Other currencies have experimented with something called Proof of Stake to solve the scaling problems but it’s not known if it can be as secure as proof of work. Facebook’s currency will probably use this method. The basic idea is that the holders of the currency vote for what happens on the network. It’s an interesting idea but it’s a very different direction than the key idea of bitcoin.
Another issue is that governments rely on setting interest rates to manage their economies. If an internet currency starts to take off that will limit their ability to do this. At that point you can expect major pushback.
And also, the cryptocurrency ecosystem is a huge target for hackers. Millions of dollars have been stolen over the last decade. Brilliant attacks have been created to exploit the tiniest flaws in code. The phone numbers of crypto enthusiasts have been hacked and their wallets drained. Hackers have even embedded malware on websites or servers to mine cryptocurrency on computers they don’t control. Facebook will need an iron clad security process if it wants to succeed.