You are missing an important they in the list of why these empty promises are made: keeping up the value of the property. There's an implicit, and often also an explicit (e.g. in the form of the founder becoming an employee) agreement that the seller won't talk down the value of what they just sold. Claiming that all will be well, despite Facebook, was very much in the interest of Facebook. Including the fact that the promise was made by someone who'd most likely be gone before the promise stopped being true.