> The only good solution would be if all messaging apps used the same protocol so everyone could be reached.

That's exactly what XMPP was created to solve, an open standard that could be implemented by anyone. For a while it even looked like there was a chance for that to work out. Whatsapp, Google Talk, Cisco Jabber, and some others used to be based on XMPP.

Unfortunately it didn't quite pan out that way.