Users were all registered by default to an email account at their ISP. All of them could send emails back and forth to people inside and outside their ISP. When

Users were all registered by default to an email account at their ISP. All of them could send emails back and forth to people inside and outside their ISP. When they started to suck, they registered new addresses on specialized email providers, and they kept their contact list with them, so emails could still be used. Because emails was pretty much the first way of communicating and because of how common it is it won't disappear soon.