However, those browsers or operating systems are written by companies. See Apple and iPhone OS for a recent example. Building on top of anything is risky unles

However, those browsers or operating systems are written by companies. See Apple and iPhone OS for a recent example. Building on top of anything is risky unless the platform (whatever that be - OS, Facebook, Twitter) provider gives you some guarantees (and you have enough trust in them that they will stick to those guarantees). If you build your app on top of someone's service and the terms of use say "we reserve the right to change anything", it's like loading a shotgun, giving it to the service provider, and hoping they'll be nice to you. If you don't/can't accept that risk, don't build on top of them.