I think the example of it working like email is the most intuitive for most people. We can all understand that @yahoo.com and @gmail.com interact with eachother despite being different service.
I would also make 1 or 2 recommendations of an instance you like because the overwhelming choice is too much for people. Imagine if you never heard of gmail or yahoo or outlook. Why choose those over @abcjoe.com? When @abcjoe.com end up sucking they'll probably just think all email sucks.