For a while there was actually an issue with iMessage where even after you switched to an Android, iMessage would continue to direct your friends' iPhones to contact you via iMessage instead of SMS. So if you left the iPhone platform, all of your iPhone-owning friends would be unable to text you.

Google Hangouts seemed to be a step in this direction as well, with SMS integration, it pushes users to use their proprietary protocol instead where possible. But it looks like Google has backed off of it since then.