The longest lived and most used services are always federated and have open standards, like SMS and email. Like you say, everyone has SMS and everyone has email, so they're the defacto standards for communicating with people you're not quite familiar with.

This is why FB chat, whatsapp, etc will never gain the traction that these services do -- putting time and effort into making them work is just a fool's errand until they open up the network to third party clients and make the specs an open standard.