Fractional Reserve Banking

Fractional Reserve Banking is the practice whereby a bank holds reserves that are a fraction of its on-demand liabilities. It contrasts with a Full Reserve Banking system in which banks must hold reserves for 100% of on-demand liabilities.

Holding only a fraction of on-demand liabilities means that in the event of a bank-run the bank is mathematically unable to meet its obligations.

Put simply: a fractional reserve bank (or exchange) faced with a bank run will not be able to give back 100% of their bitcoins to users and will go bankrupt.

January 3rd Proof of Keys day

Every year on January 3rd bitcoiners are encouraged to move their bitcoins to wallets under their full control. By withdrawing from exchanges and other custodians, fractional reserve, "paper bitcoin", ponzis and other frauds are exposed. This makes Bitcoin stronger by removing weak players from the system.

Proof of keys day is based on the "not your keys, not your Bitcoin" and "Don't trust, verify." Bitcoin maxims.