They invested $900 million[1] in 2015, and it is worth $94.1 billion today. Why sell? They also likely have restrictions on selling post-IPO, but I don't think you can trigger a "bank run" on a stock anyway; the price will just keep adjusting as you sell/buy until it gets absurd.
A bank run happens when everyone tries to withdraw all their money at the same time and the bank runs out of cash. Not really possible in the stock market where companies literally can create/destroy shares and there is a whole secondary pricing layer to it.
1. https://www.reuters.com/business/finance/alphabets-spacex-be...