It definitely could work, but who will make it work? I would really like to see my wife's facebook posts without having a facebook account (or just looking at her iPad which is what I currently do...) and she may be interested in my posts elsewhere.

I can see how it should be easy to send content from foo#twitter.com to bar#facebook.com as individual messages, format issues not withstanding, if foo and bar have agreed to correspond. But which end would manage the existence of that relationship and push/pull the data?

Maybe it needs third-party brokers to do it?

Interestingly I can see some parallels here with the current status of banking in the UK. After centuries of operation as "closed systems" there are now rules to force banks to provide APIs so that new entrants can offer inter-operation services. Maybe in a hundred years social-media companies will be forced to do the same? ;-)