To start a bank you need sufficient capital
If you have that, you can start a bank, as plain and simple as it gets. This is certainly not about "no one is allowed to start a bank". Apart from that, I don't see what it has to do with your point. Further, it is not my idea that big corporations can and do establish their own "pet" banks (I have provided you a link). In fact, many international or just large enough companies which started as pure production (or whatever) firms are now part of large financial groups which include banks to serve the interests of these groups. The fact that Facebook doesn't have a bank means that it is not a corporation (which it is not) or it doesn't need a bank, just as simple. I don't understand why you continue this futile argument
Gazprombank was founded in 1990. Its not a new bank. Russia being one of the few countries not run by international bankers, it is probably one of the few countries which would allow someone to start a relatively new bank.
Can you name a bank that was recently founded by a large corporation as you claim?
It is illegal to start a bank in most countries that are run by central banks. There's a reason why no one goes into the banking business despite the technology being used by most banks being old, obsolete and outdated.