Nothing would prevent it. Some might argue it might be inevitable. In order for any network or group to start offering banking and payments, they would need to get licensed.
Licensing in itself might not be a very difficult thing to do, but what would be required is tightening of the loose ends. KYC (Know Your Customer) requirements would be much more stringent. Security would be paramount and the provider would have to open their systems to auditors to assess the same.
Multi-jurisdiction licensing would be required if money/value is traversing across the borders. In some cases, value transfers would only be unidirectional because of currency flow controls, etc.
With all this in mind, the social network would have to assess how much money can they make from this? Would monopoly control authorities in various countries come after them? Would there be enough of a stimulus for users to switch? How will the platform guarantee safety of funds and accompany data, when in most cases such platforms have known to have had breaches, etc.
Nothing prevents the social network from becoming a banking and a payments network, the question is, do they want to go in that direction?