The configuration is never over with email, because every time I sign up for something, I've got more filters to set up. With instant messaging? There was never a problem to begin with

No it just read my pre-existing social network graph. And then very accurately predicts who I might want to add to that graph. But practically speaking it comes out to the same thing - I have never had to create a messaging whitelist.

While email and email tooling is completely ignorant of my multiple social network graphs that are stored in various places on the Internet.