Reminds me of a simpler time when messaging was universal, not a Machiavellian marketing scheme to generate social friction in order to sell more phones.
The problem is that the legal system is misused to prevent adversarial interoperability.
Adversarial interoperability was never explicitly welcomed by gatekeepers, but back then nobody would think of suing over it, nor prosecuting such a case.
Nowadays, copyright law is used as a reason to shut those things down, even when the copyrighted content is actually third-party user-generated content or content you do have a license to access since it's freely accessible using the official client.
IRC
ICQ
AIM
MSN Messenger
Y! Messenger
Etc
You not only needed to know the ID but also the service to send it on, which is exactly the same situation today. Except for email