This is part of why I like the promise of WebFinger.

To Chris, the @reply would "just work" regardless of what network I was on. Assuming the other party supported the above protocols, this could have worked to,,,,, etc. And the protocols themselves are easy, likely only a simple extension of endpoints these networks are already exposing. And not only is this process fully decentralized (i.e., there doesn't need to be a Google in the middle), it delegates very well, so that domain owners wouldn't have to implement it themselves -- but they could later if they wanted to.